Routing Protocol in detail

Objectives Introduction of EIGRP, Features of EIGRP, Tables of EIGRP, Metric of EIGRP
 Configuration of EIGRP, Summarization with EIGRP
 EIGRP unequal cost load balancing, Variance, Authentication
 EIGRP stub router, Configure EIGRP Stub Network.
 EIGRP Summary
 Understanding OSPF Fundamentals, Configuring OSPF within Single Area, Cost
 Understanding the differences between OSPF Network Types, Using OSPF Across
Multiple Area- Features, Router Types
 Configure OSPF virtual Link & Summarization
 "Link- State Advertisements, Different Types Of Areas, Propagation of LSAs, Special
OSPF Area"
 Configure OSPF Stub Areas & Authentication
 OSPF Summary
 Introduction to BGP, Context for the BGP, BGP Route Stability, BGP Operation Basics,
When to use BGP, BGP States
 Configuring Default Routing with BGP, Configure BGP using AS_PATH attribute
 "BGP Attributes, Controlling BGP Route Selection using the Weight, Local-Preference,
MED"
 Configure IBGP and EBGP using Local Preference & MED attributes.
 WorkShop

Understanding
EIGRP Terminology
and Operation

 EIGRP is a Cisco-proprietary distance-vector protocol with link-state features.
 EIGRP features include:
• Fast convergence
• Partial updates
• Multiple network layer support
• Use of multicast and unicast communication
• Variable-length subnet masking (VLSM) support
• Seamless connectivity across all data link layer protocols and topologies
• By default, it performs automatic route summarization at major network boundaries
(can be disabled) but can also be configured to summarize on interfaces.
EIGRP Features

 Neighbor table
 Topology table
 Routing table
 Advertised Distance (AD)
 Feasible Distance (FD)
 Successor
 Feasible successor (FS)
 Passive Versus Active Routes
EIGRP Terminology

 Neighbor table
• Contains EIGRP neighbor addresses and the interface through which they can be reached.
 Topology table
• Contains all destinations advertised by neighboring routers.
 Routing table
• Contains EIGRP successor routes.
EIGRP Tables

 Advertised Distance (AD)
• Advertised distance (AD), also referred to as the Reported Distance, is the cost between the
next-hop router and the destination.
 Feasible Distance (FD)
• Feasible distance (FD) is the cost between the local router and the next-hop router plus the
next-hop router’s AD to the destination network.
AD versus FD

 Successor
◦ A successor is a neighboring router that has a least-cost path to a destination (the lowest FD)
that is guaranteed not to be part of a routing loop.
◦ Successor routes are offered to the routing table to be used for forwarding packets.
◦ Multiple successors can exist if they have the same FD.
 Feasible successor (FS)
◦ A feasible successor is a neighbor that is closer to the destination, but it is not the least-cost
path.
◦ A feasible successor ensures a loop-free topology because it must have an AD less than the FD of
the current successor route.
◦ Feasible successors are selected at the same time as successors but are kept in the topology
table as backups to the successor routes.
◦ The topology table can maintain multiple feasible successors for a destination.
Successor and Feasible Successor

 Passive Route
• A route is considered passive when the router is not performing recomputation on that route.
• Passive is the operational, stable state.
 Active route
• A route is active when it is undergoing recomputation.
Passive versus Active Routes

 Reliable Transport Protocol (RTP)
◦ Responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors.
 Neighbor discovery/recovery mechanism
◦ Enables EIGRP routers to dynamically learn when their neighbors become unreachable or
inoperative by periodically sending small hello packets.
 Protocol-dependent modules (PDMs)
◦ Responsible for network layer protocol-specific requirements such as IP, IPv6, AppleTalk, and
Novell NetWare.
 DUAL finite-state machine
◦ Diffusing Update Algorithm (DUAL) is the routing algorithm that tracks all routes advertised
by all neighbors and uses distance information, known as the composite metric, to select
efficient, loop-free paths to all destinations.
Key EIGRP Technologies

 EIGRP cannot use the services of UDP or TCP since IPX and Appletalk do not use the TCP/IP
protocol suite.
 Reliable Transport Protocol (RTP) is the Transport layer protocol uniquely used by EIGRP for the
delivery and reception of EIGRP packets.
• RTP is similar to TCP but is a Cisco proprietary.
 RTP provides reliable or unreliable service as the situation warrants.
• Reliable packets (Update, Query, Reply) require explicit acknowledgement while unreliable
packets (Hello, ACK) do not.
Reliable Transport Protocol

 EIGRP routers actively establish relationships with their neighbors.
 Adjacencies are established using small Hello packets which are sent every 5 or 60 seconds.
• If a neighbor misses 3 consecutive Hello packets then the route is considered invalid.
• Default = 15 seconds or 180 seconds.
Neighbor Discovery / Recovery

 Various routed protocols are supported through its PDMs.
• Provides independence from routed protocols.
• PDMs are modular, scalable and adaptable.
• EIGRP can adapt to new or revised routed protocols.
• PDMs protect EIGRP from painstaking revision.
 Each PDM is responsible for all functions related to its specific routed protocol.
Protocol-Dependent Modules
EIGRP maintains
individual tables for
each routed protocol.

 DUAL uses the Neighbor and Topology tables to calculate route information.
 When a link fails, DUAL looks for a feasible successor in its Neighbor and Topology tables.
• It compares all routes advertised by neighbors by using a composite metric for each route.
• Lowest-cost paths are then inserted into the routing table.
DUAL finite-state machine

EIGRP Packet
Frame Header
Frame Payload
CRCIP
Header
Protocol Number
(EIGRP = 88)
EIGRP
Header
EIGRP
Message
On a LAN, the EIGRP
packet is encapsulated
in an Ethernet frame
with a destination
multicast MAC address:
01-00-5E-00-00-0A
The destination IP address is
set to the multicast 224.0.0.10
and the EIGRP protocol field
is 88.
The EIGRP
header identifies
the type of EIGRP
packet and
autonomous
system number.
The EIGRP
message
consists of
the Type /
Length /
Value (TLV).

 EIGRP uses these 5 packet types to maintain its various tables and establish complex relationships
with neighbor routers:
• Hello
• Acknowledgment
• Update
• Query
• Reply
EIGRP Header

 EIGRP relies on Hello packets to discover, verify, and rediscover neighbor routers.
 EIGRP Hello packets are multicast to 224.0.0.10.
 Hello packets are always sent unreliably and therefore do not require acknowledgment.
Hello Packets

 Hellos are sent at a fixed (and configurable) interval, called the Hello interval.
• Hello/Hold timers do not need to match.
• To reset the Hello interval: no ip hello-interval eigrp as#
 Hello interval depends on the interface’s bandwidth.
• High bandwidth = 5 seconds
• Default interval on point-to-point serial links, multipoint circuits with bandwidth greater
than T1, and LANs.
• Low Bandwidth = 60 seconds
• Default interval on T1 or less multipoint WAN circuits.
 On hearing Hellos, a router creates a neighbor table and the continued receipt of Hellos maintains
the table.
 Hold time is the maximum amount of allowed time that Hellos are not heard from a neighbor.
• Three times the Hello Interval:
• Low Bandwidth (3 x 60 sec.) = 180 seconds
• High bandwidth (3 x 5 sec.) = 15 seconds
Hello Packets

 Are used to indicate receipt of any EIGRP packet during a "reliable" (i.e., RTP) exchange.
• To be reliable, a sender's message must be acknowledged by the recipient.
 Acknowledgment packets are:
• Dataless Hello packets.
• Unicast.
Acknowledgement Packets

 After the local router discovers a new neighbor, update packets are sent to the new neighbor.
 Update packets are also used when a router detects a topology change.
• The router sends a multicast Update packet to all neighbors, alerting them to the change.
 All Update packets are sent reliably.
Update Packets
Update packet
• Initially sent after a new neighbor is discovered.
• Sent when a topology change has been detected.

 Query and Reply packets are sent when a destination has no feasible successors.
 Both packet types are sent reliably.
 A Query packet is multicasted to other EIGRP routers during the route re-computation process.
• Query packets are always multicast.
 A Reply packet is used to respond to a query to instruct the originator not to recompute the route
because feasible successors exist.
• Reply packets are always unicast.
Query and Reply Packets

Packet Types
Packet Type Use
Hello Used to discover other EIGRP routers in the
network.
Acknowledgement Used to acknowledge the receipt of any EIGRP
packet.
Update Convey routing information to known destinations.
Query Used to get specific information from a neighbor
router.
Reply Used to respond to a query.

 EIGRP selects primary (successor) and backup (feasible successor) routes and injects those into the
topology table.
 The primary (successor) routes are then moved to the routing table.
EIGRP Operations
IP EIGRP Neighbor Table
Neighbor IP Address Local router exit
interface to neighbor
IP EIGRP Topology Table
Destination 1 FD / AD via each neighbor
IP Routing Table
Destination 1 Best route
List of directly connected adjacent EIGRP
neighbor routers and the local interface to exit
to reach it.
List of all routes learned from each EIGRP
neighbor and identifies successor routes and
feasible successor routes.
List of the best (successor) routes from the
EIGRP topology table and other routing
processes.

Example: EIGRP Tables
Router C’s tables:

R1# show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.1.102 Se0/0/1 11 00:07:22 10 2280 0 5
R1#
EIGRP Neighbor Table
Lists the order in
which a peering
session was
established with
the specified
neighbor, starting
with 0.
Neighbor’s
IP address
Local interface
receiving EIGRP
Hello packets.
Seconds remaining before declaring neighbor
down.
The current hold time and is reset to the
maximum hold time whenever a Hello packet
is received.
SRTT (Smooth Round Trip Timer) and RTO (Retransmit Interval) are used by
RTP to manage reliable EIGRP packets.
SRTT indicates how long it takes for this neighbor to respond to reliable
packets.
RTO indicates how long to wait before retransmitting if no ACK is received.
Queue count
should always be
zero otherwise
there’s
congestion on the
link.
The sequence
number of the
last update,
query, or reply
packet that was
received from
this neighbor.
Amount of time
since this
neighbor was
added to the
neighbor table.

R1# show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(192.168.1.101)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 172.17.0.0/16, 1 successors, FD is 40514560
via 192.168.1.102 (40514560/28160), Serial0/0/1
R1#
EIGRP Topology Table
Indicates if the
route is in passive
or active state.
Destination network. Number of
successors
Next-hop address
for successor.
Outbound interface
to reach the network.
Feasible distance (FD)
to the destination
Feasible
distance (FD)
to the
destination
Advertised
distance (AD)
from the
successor

EIGRP Routing Table
EIGRP
route
Destination
network
R1# show ip route
<output omitted>
Gateway of last resort is not set
D 172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:02:22, Serial0/0/1
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 172.16.0.0/16 is a summary, 00:31:31, Null0
C 172.16.1.0/24 is directly connected, FastEthernet0/0
R1#
Administrative
distance
Feasible
distance
Next-hop address
to reach the
network
Time indicating
the last update
packet received
Local router exit
interface to
destination
network
Summary route automatically created as the result
of the default classful behavior of EIGRP.

 EIGRP default administrative distances
EIGRP Administrative Distance (AD)
Routes manually
summarized.
Routes redistributed into
EIGRP.

DUAL Example
(1)
10.1.1.0 /24
A
C
B
E
D
(2) (2) (1)
(1)
(1)
EIGRP FD AD Topology
10.1.1.0 /24 3 ***** Passive *****
via B 3 1 Successor
via D 4 2 Feasible Successor
via E 4 3
Router C
10.1.1.0 /24 3 ***** Passive *****
via D 3 2 Successor
via C 4 3
Router E
10.1.1.0 /24 2 ***** Passive *****
via B 2 1 Successor
via C 5 3
Router D

DUAL Example
(1)
10.1.1.0 /24
A
C
B
E
D
(2) (2) (1)
(1)
10.1.1.0 /24 3 ***** Passive *****
via B 3 1 Successor
via D 4 2 Feasible Successor
via E 4 3
Router C
10.1.1.0 /24 3 ***** Passive *****
via D 3 2 Successor
via C 4 3
Router E
10.1.1.0 /24 -1 ***** ACTIVE ******
via E (Q) Query
via C 5 3 (Q) Query
Router D
Q
Q Q = Query

DUAL Example
(1)
10.1.1.0 /24
A
C
B
E
D
(2) (2) (1)
(1)
10.1.1.0 /24 3 ***** Passive *****
via B 3 1 Successor
via D
via E 4 3
Router C
10.1.1.0 /24 -1 ***** ACTIVE ******
via D
via C 4 3 (Q) Query
Router E
10.1.1.0 /24 -1 ***** ACTIVE ******
via E (Q) Query
via C 5 3
Router D
R
Q
Q = Query
R = Reply

DUAL Example
(1)
10.1.1.0 /24
A
C
B
E
D
(2) (2) (1)
(1)
10.1.1.0 /24 3 ***** Passive *****
via B 3 1 Successor
via D
via E
Router C
10.1.1.0 /24 4 ***** Passive *****
via C 4 3 Successor
via D
Router E
10.1.1.0 /24 -1 ***** ACTIVE ******
via E (Q) Query
via C 5 3
Router D
R
Q = Query
R = Reply

DUAL Example
(1)
10.1.1.0 /24
A
C
B
E
D
(2) (2) (1)
(1)
10.1.1.0 /24 3 ***** Passive *****
via B 3 1 Successor
via D
via E
Router C
10.1.1.0 /24 4 ***** Passive *****
via C 4 3 Successor
via D
Router E
10.1.1.0 /24 5 ***** Passive *****
via C 5 3 Successor
via E 5 4 Successor
Router D
R
Q = Query
R = Reply

DUAL Example
(1)
10.1.1.0 /24
A
C
B
E
D
(2) (2) (1)
(1)
10.1.1.0 /24 3 ***** Passive *****
via B 3 1 Successor
via D
via E
Router C
10.1.1.0 /24 4 ***** Passive *****
via C 4 3 Successor
via D
Router E
10.1.1.0 /24 5 ***** Passive *****
via C 5 3 Successor
via E 5 4 Successor
Router D

 EIGRP uses a composite metric which can be based on the following metrics:
◦ Bandwidth
◦ Delay
◦ Reliability
◦ Load
 Only Bandwidth and Delay are used by default.
Note: It is often incorrectly stated that EIGRP can also use the smallest MTU in the path. In actual fact,
the MTU is included in the EIGRP routing update, but is not actually used in the metric calculation.
EIGRP Metric Calculation

 EIGRP uses the slowest bandwidth (BW) in its metric calculation.
• Calculated BW = reference BW / slowest BW (kbps)
 The value of the bandwidth may or may not reflect the actual physical bandwidth of the interface.
• For example, most serial interfaces use the default bandwidth value of 1.544 Mbps but this may
not accurately reflect the links actual bandwidth.
 Because both EIGRP and OSPF use bandwidth in default metric calculations, a correct value for
bandwidth is very important to the accuracy of routing information.
◦ If the actual bandwidth of the link differs from the default bandwidth value, then the bandwidth
value should be modified.
 To modify the bandwidth value, use the bandwidth interface command.
Note: The bandwidth command does NOT change the physical bandwidth of the link.
EIGRP Bandwidth

EIGRP Delay
 Delay is a measure of the time it takes for a
packet to traverse a route.
• EIGRP uses the cumulative sum of all
outgoing interfaces.
• Calculated Delay = the sum of outgoing
interface delays / 10
 The delay (DLY) metric is a static value based
on the type of link to which the interface is
connected and is expressed in microseconds.

 Reliability (not a default EIGRP metric) is a measure of the likelihood that a link will fail.
• Measure dynamically & expressed as a fraction of 255.
• The higher the fraction the better the reliability
 Load (not a default EIGRP metric) reflects how much traffic is using a link
• Number is determined dynamically and is expressed as a fraction of 255
• The lower the fraction the less the load on the link
 These optional criteria can be used but are not recommended, because they typically result in
frequent recalculation of the topology table.
Other EIGRP Metrics

EIGRP Composite Metric Calculation
 The EIGRP composite metric formula consists of values K1 through K5, known as EIGRP metric
weights.
• By default, only K1 (bandwidth) and K3 (delay) are set to 1.
• K2 (load), K4 (reliability), and K5 (MTU) are set to 0.
 K values can be changed with the EIGRP router command:
Router(config-router)# metric weights tos k1 k2 k3 k4 k5

 EIGRP neighbors cannot use mismatched metric values.
• All EIGRP neighbors must use the same metrics.
• Metrics can be altered using the metric weights command.
Mismatched K Values

EIGRP Metric Calculation Example
Slowest bandwidth:
Plus the sum of the delays

 Bandwidth = 10,000,000 / 1024 = 9765 * 256 = 2499840
EIGRP Bandwidth Calculation Example

 Delay = 20,000 / 10 + (100 / 10) * 256 = 514560
EIGRP Delay Calculation Example

 EIGRP Metric = 2499840 + 514560 = 3014400
EIGRP Metric Calculation Example

Planning EIGRP
Routing
Implementations

 Prior to deploying an EIGRP routing solution, the following should be considered:
• IP addressing plan
• Network topology
• EIGRP traffic engineering
 Once the requirements have been assessed, the implementation plan can be created.
Planning to Deploy EIGRP

 The information necessary to implement EIGRP routing includes the following:
• The IP addresses to be configured on individual router interfaces
• The EIGRP AS number, used to enable EIGRP.
• A list of routers and interfaces on which EIGRP is to be enabled.
• Metrics that need to be applied to specific interfaces, or EIGRP traffic engineering.
 In the implementation plan, EIGRP the tasks include the following:
• Enabling the EIGRP routing protocol.
• Configuring the proper network statements.
• Optionally configuring the metric to appropriate interfaces.
Implementing EIGRP

 After implementing EIGRP, verification should confirm proper deployment on each router.
 Verification tasks include verifying:
• The EIGRP neighbor relationships.
• That the EIGRP topology table is populated with the necessary information.
• That IP routing table is populated with the necessary information.
• That there is connectivity in the network between routers and to other devices.
• That EIGRP behaves as expected in a case of a topology change, by testing link failure and router
failure events.
Verifying EIGRP

 After a successful EIGRP deployment, the solution and verification process and results should be
documented for future reference.
 Documentation should include:
• A topology map
• The IP addressing plan
• The AS number used
• The networks included in EIGRP on each router
• Any special metrics configured
Documenting

Configuring and
Verifying EIGRP

Enable EIGRP Routing
 Define EIGRP as the IP routing protocol.
Router(config)#
router eigrp autonomous-system-id
 To exchange routing updates, EIGRP routers must have the same autonomous system ID.

Identify EIGRP Networks
 Define EIGRP networks to advertise to EIGRP neighbors.
Router(config-router)#
network network [mask]
 The network parameter can be a network, a subnet, or the address of a directly connected interface.
 The mask is a wildcard mask (inverse mask) used to determine how to interpret the address.
• The mask has wildcard bits, where 0 is a match and 1 is “don’t care.”
• For example, 0.0.255.255 indicates a match in the first 2 octets.

 Most EIGRP references state that the wildcard mask is required.
 However, since IOS 12.0(4)T, the mask argument can actually be configured using wild card bits or a
regular subnet mask.
 For example, either format could be used to configure the 10.10.10.0 network:
network 10.10.10.0 0.0.0.3
or
network 10.10.10.0 255.255.255.252
Note on EIGRP Masks

Define the Interface Bandwidth
 Defines the interface’s bandwidth (optional).
Router(config-if)#
bandwidth kilobits
 The kilobits parameter indicates the intended bandwidth in kbps.
 For example, to set the bandwidth to 512,000 bps, use the bandwidth 512 command.
 The configured bandwidth is used by routing protocols in the metric calculation.
 The command does not actually change the speed of the interface.

Enable / Disable Automatic Summarization
 By default, EIGRP automatically summarizes subnets.
auto-summary
 This makes EIGRP behave like a classful routing protocol and therefore summarizes subnets on the
classful boundary.
 Automatic summarization can be disabled using the no auto-summary router configuration command.

Configuring EIGRP Example: Classful
R1(config)# interface Fa0/0
R1(config-if)# ip address 172.16.1.1 255.255.255.0
R1(config-if)# no shut
R1(config-if)# interface S0/0/0
R1(config-if)# bandwidth 64
R1(config-if)# exit
Classful configuration example:
R2(config-if)# exit
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
S0/0/1
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102 .1
.1.1

Configuring EIGRP Example: Classful
R1(config)# router eigrp 100
R1(config-router)# network 192.168.1.96
R1(config-router)#
*Jul 26 10:02:25.963: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 192.168.1.101 (Serial0/0/0) is
up: new adjacency 172.17.2.0
R2(config-router)#
R2#
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
S0/0/1
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102 .1
.1.1

Verifying EIGRP Example
R1# show running-config | section router eigrp
router eigrp 100
network 172.16.0.0
network 192.168.1.0
auto-summary
R1# show ip route
<output omitted>
D 172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:24:02, Serial0/0/0
C 192.168.1.96/27 is directly connected, Serial0/0/0
D 192.168.1.0/27 [90/41024000] via 192.168.1.102, 00:16:56, Serial0/0/0
R1#
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
S0/0/1
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102 .1
.1.1

router eigrp 100
network 172.17.0.0
network 192.168.1.0
auto-summary
R2# show ip route
<output omitted>
D 172.16.0.0/16 [90/40514560] via 192.168.1.101, 00:13:26, Serial0/0/0
R2#
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
S0/0/1
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102 .1
.1.1

R2# show ip protocols
Routing Protocol is "eigrp 100"
<output omitted>
Automatic network summarization is in effect
Automatic address summarization:
192.168.1.0/24 for FastEthernet0/0
Summarizing with metric 40512000
172.17.0.0/16 for Serial0/0/0, Serial0/0/1
Maximum path: 4
Routing for Networks:
172.17.0.0
192.168.1.0
Routing Information Sources:
<output omitted>
R2#
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
S0/0/1
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102 .1
.1.1

Configuring EIGRP Example: Classless
Classless configuration example:
R2(config)# no router eigrp 100
R2(config-router)# network 192.168.1.96 0.0.0.31
R2(config-router)# end
R2# show run | section router eigrp
router eigrp 100
network 172.17.2.0 0.0.0.255
network 192.168.1.96 0.0.0.31
auto-summary
R2#
Fa0/0Fa0/0
172.17.2.0 /24
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
64 kbps
192.168.1.96 /27
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1
R1(config)# no router eigrp 100
R1# show run | section router eigrp
router eigrp 100
network 172.16.1.0 0.0.0.255
network 192.168.1.96 0.0.0.31
auto-summary
R1#

<output omitted>
172.17.0.0/16 for Serial0/0/0
Maximum path: 4
172.17.2.0/24
192.168.1.96/27
Gateway Distance Last Update
(this router) 90 00:00:06
192.168.1.101 90 00:00:26
Distance: internal 90 external 170
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
S0/0/1
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102 .1
.1.1

Verifying EIGRP: show ip protocols
<output omitted>
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 100
EIGRP NSF-aware route hold timer is 240s
172.16.0.0/16 for Serial0/0/0
Maximum path: 4
172.16.1.0/24
192.168.1.96/27
(this router) 90 00:08:56
192.168.1.102 90 00:07:59
Distance: internal 90 external 170
Verify routing protocol information on the router.

Verifying EIGRP: show ip eigrp neighbors
(sec) (ms) Cnt Num
0 192.168.1.102 Se0/0/0 11 00:09:17 22 2280 0 5
R1#
EIGRP uses the Neighbor table to list adjacent routers.

Verifying EIGRP: show ip eigrp topology
R1# show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(192.168.1.101)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
via Connected, Serial0/0/0
via Summary (40512000/0), Null0
via Summary (28160/0), Null0
via 192.168.1.102 (40514560/28160), Serial0/0/0
via Connected, FastEthernet0/0
R1#

Verifying EIGRP: show ip route eigrp
R1# show ip route eigrp
D 172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:10:18, Serial0/0/0
R1#
R1# show ip route
<output omitted>
D 172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:10:35, Serial0/0/0
R1#
Verify that the router recognizes EIGRP routes.

Verifying EIGRP: show ip eigrp interfaces
R1# show ip eigrp interfaces
IP-EIGRP interfaces for process 100
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/0/0 1 0/0 22 10/380 468 0
Fa0/0 0 0/0 0 0/1 0 0
R1#
Verify EIGRP configured interfaces.

Verifying EIGRP: show ip eigrp traffic
R1# show ip eigrp traffic
IP-EIGRP Traffic Statistics for AS 100
Hellos sent/received: 338/166
Updates sent/received: 7/7
Queries sent/received: 0/0
Replies sent/received: 0/0
Acks sent/received: 2/2
SIA-Queries sent/received: 0/0
SIA-Replies sent/received: 0/0
Hello Process ID: 228
PDM Process ID: 226
IP Socket queue: 0/2000/1/0 (current/max/highest/drops)
Eigrp input queue: 0/2000/1/0 (current/max/highest/drops)
R1#
Verify EIGRP traffic information.

Verifying EIGRP: debug eigrp packets
R2# debug eigrp packets
*Jul 26 10:51:24.051: EIGRP: Sending HELLO on Serial0/0/0
*Jul 26 10:51:24.051: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*Jul 26 10:51:24.111: EIGRP: Sending HELLO on FastEthernet0/0
*Jul 26 10:51:26.667: EIGRP: Received HELLO on Serial0/0/0 nbr 192.168.1.101
*Jul 26 10:51:26.667: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/re
ly 0/0
ly 0/0
ly 0/0
Traces transmission and receipt of EIGRP packets.

EIGRP Passive-Interface
 Prevent EIGRP updates out a specified router interface.
passive-interface type number [default]
 Set a particular interface or all router interfaces to passive.
 The default option sets all router interfaces to passive.
 For EIGRP, the command:
 Prevents neighbor relationships from being established.
 Routing updates from a neighbor are ignored.
 Allows a subnet on a passive interface to be announced in EIGRP

Passive-Interface Example
R1(config-router)# passive-interface fa0/0
R1(config-router)#
R2(config-router)#
Alternate configuration:
R1(config-router)# passive-interface default
R1(config-router)# no passive-interface S0/0/0
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
Internet
192.168.1.0 /27
172.17.2.0 /24
64 kbps
192.168.1.96 /27
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1

 To propagate a default route in EIGRP, use either the:
ip default-network network-number global configuration command.
Or
ip route 0.0.0.0 0.0.0.0 next-hop | interface router configuration
command.
 Once configured, the default route has to be propagated into the EIGRP AS.
Propagating a Default Route

ip default-network Command
 Configures a router to advertise a network as the gateway of last resort.
Router(config)#
ip default-network network
 Other routers use their next-hop address to the advertised network as their default route.
 There is no parameter to specify the subnet mask therefore the command can only be used to
advertise a classful network.
 The specified network must be reachable before it is configured.
 If the specified network is reachable through:
 EIGRP, then the default route is propagated automatically to other EIGRP routers in the AS.
 A static route, then the static route must be redistributed into EIGRP.

ip default-network Example
R2# show ip route
<output omitted>
D 172.16.0.0/16 [90/40514560] via 192.168.1.101, 02:27:56, Serial0/0/0
192.168.1.0/27 is subnetted, 2 subnets
C 192.168.1.96 is directly connected, Serial0/0/0
R2#
R1# show ip route
<output omitted>
<output omitted>
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
64 kbps
EIGRP AS 100
Internet
172.31.0.0 /16192.168.1.96 /27
172.17.2.0 /24
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1
192.168.1.0 /27
.2

Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
64 kbps
EIGRP AS 100
Internet
172.31.0.0 /16192.168.1.96 /27
172.17.2.0 /24
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1
192.168.1.0 /27
.2
R2(config)# ip route 172.31.0.0 255.255.0.0 192.168.1.2
R2(config)# do ping 172.31.0.0
<output omitted>
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/28/28 ms
R2(config)# ip default-network 172.31.0.0
R2(config-router)# redistribute static
R2#

R2# show ip route
<output omitted>
Gateway of last resort is 192.168.1.2 to network 172.31.0.0
<output omitted>
S* 172.31.0.0/16 [1/0] via 192.168.1.2
R2#
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
64 kbps
EIGRP AS 100
Internet
172.31.0.0 /16192.168.1.96 /27
172.17.2.0 /24
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1
192.168.1.0 /27
.2
R1# show ip route
<output omitted>
<output omitted.
D*EX 172.31.0.0/16 [170/41024000] via 192.168.1.102, 00:00:20, Serial0/0/0
R1#

ip route 0.0.0.0 0.0.0.0 next-hop | interface Command
 Configures a router to advertise a default route as the gateway of last
resort.
Router(config)#
ip route 0.0.0.0 0.0.0.0 interface | next-hop
 The choice of parameter affects the next selection of commands.
 If the interface parameter is used, then only the network 0.0.0.0
needs to also be entered.
 If the next-hop parameter is used, then the network 0.0.0.0 and
the redistribute static command must be configured.

ip route 0.0.0.0 0.0.0.0 interface Example
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
64 kbps
EIGRP AS 100
Internet
172.31.0.0 /16192.168.1.96 /27
172.17.2.0 /24
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1
192.168.1.0 /27
.2
R2(config)# ip route 0.0.0.0 0.0.0.0 S0/0/1
R2(config-router)# do show ip route
<output omitted>
D 172.16.0.0/16 [90/40514560] via 192.168.1.101, 03:13:25, Serial0/0/0
S* 0.0.0.0/0 is directly connected, Serial0/0/1
R2(config-router)#

ip route 0.0.0.0 0.0.0.0 next-hop Example
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
64 kbps
EIGRP AS 100
Internet
172.31.0.0 /16192.168.1.96 /27
172.17.2.0 /24
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1
192.168.1.0 /27
.2
R2(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.2
R2(config-router)# redistribute static
R2(config-router)# do show ip route
<output omitted>
D 172.16.0.0/16 [90/40514560] via 192.168.1.101, 02:53:48, Serial0/0/0
S* 0.0.0.0/0 [1/0] via 192.168.1.2
R2(config-router)#

 EIGRP automatically summarizes routes at a major network boundary by default.
• Due to the pre-configured auto-summary router configuration command.
• In most cases, auto summarization is a good thing as it keeps routing tables as compact as
possible.
• Sometimes it’s not a good thing such as when there is a discontiguous subnetwork.
 Typically for routing to work properly, auto-summarization should be disabled using the no
auto-summary router configuration command.
EIGRP Route Summarization

Summarization in Discontiguous Networks
R1 R2
10.10.10.0 /24
EIGRP AS 100
192.168.1.96 /30 10.20.20.0 /24
 EIGRP Update: Connected to 10.0.0.0 /8
router eigrp 100
passive-interface FastEthernet0/0
network 10.10.10.0 0.0.0.255
network 192.168.1.96 0.0.0.31
auto-summary
<output omitted>
10.0.0.0/8 for Serial0/0/0
Maximum path: 4
10.10.10.0/24
192.168.1.96/27
Passive Interface(s):
FastEthernet0/0
<output omitted>

Summarization in Discontiguous Networks
R2# show ip route
<output omitted>
S* 0.0.0.0/0 [1/0] via 192.168.1.2
R2#
R1 R2
10.10.10.0 /24
EIGRP AS 100
192.168.1.96 /30 10.20.20.0 /24
R2 ignores the R1 update
because it is already
connected to the classful
10.0.0.0/8 network.

 Notice that the summarized route (10.0.0.0/8) has an entry pointing to null0.
• Null0 is automatically added to the table and are called summary routes.
• Null 0 is a directly connected, software-only interface.
• The use of the null0 interface prevents the router from trying to forward traffic to other routers
in search of a more precise, longer match.
Null 0
R2# show ip route
<output omitted>
S* 0.0.0.0/0 [1/0] via 192.168.1.2
R2#

Disabling Automatic Summarization
R1 R2
10.10.10.0 /24
EIGRP AS 100
192.168.1.96 /30 10.20.20.0 /24
R1(config-router)# no auto-summary
R1(config-router)#
*Jul 26 22:14:07.183: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 192.168.1.102
(Serial0/0/0) is resync: summary configured
Routing Protocol is "eigrp 100“
<output omitted>
Automatic network summarization is not in effect
Maximum path: 4
10.10.10.0/24
192.168.1.96/27
<output omitted>

Disable Automatic Summarization
R1 R2
10.10.10.0 /24
EIGRP AS 100
192.168.1.96 /30 10.20.20.0 /24
R2(config)# no auto-summary
R2(config)# end
R2# show ip route
<output omitted>
C 10.20.20.0 is directly connected, FastEthernet0/0
D 10.10.10.0 [90/40514560] via 192.168.1.101, 00:05:21, Serial0/0/0
S* 0.0.0.0/0 [1/0] via 192.168.1.2
R2#

 Earlier distance vector protocols were unable to create summary routes other than the classful
boundaries or /8, /16/ or /24.
 To address this shortcoming, EIGRP added the ip summary-address eigrp interface
configuration command.
◦ The command is used to create one or more summary routes within a network on any bit
boundary (as long as a more specific route exists in the routing table).
 IP EIGRP summary routes are given an administrative distance value of 5.
◦ Standard EIGRP routes receive an administrative distance of 90
◦ External EIGRP routes receive an administrative distance of 170.
Summarizing on an Interface

ip summary-address eigrp
 Manually create a summary route at an arbitrary bit boundary.
Router(config-if)#
ip summary-address eigrp as-number address mask [admin-distance]
Parameter Description
as-number The number of the EIGRP AS is identified.
address
The IP address being advertised as the summary
address. This address does not need to be aligned
on Class A, B, or C boundaries.
mask The IP subnet mask used to create the summary
address.
admin-distance (Optional) Administrative distance. A value from 0 to
255.

EIGRP Route Summarization
Fa0/0Fa0/0
R1 R2
10.10.10.0 /24
EIGRP AS 100
WAN
192.168.3.1
S0/0/0
10.10.20.0 /24
10.0.0.0 /8
R3
R3(config)# interface S0/0/0
R3(config-if)# ip summary-address eigrp 1 10.10.0.0 255.255.0.0
R3(config-if)# exit
<output omitted>
Automatic network summarization is not in effect
Address Summarization:
10.10.0.0/16 for Serial0/0/0
<output omitted>

 Routes with a metric equal to the minimum metric are installed in the routing table.
• Referred to as “equal-cost load balancing”.
• All IP routing protocols on Cisco routers can perform equal-cost load balancing.
 The maximum-paths maximum-path command can be used to allow up to 6 equal-cost paths.
• Default is 4.
• Setting the maximum-path option to 1 disables load balancing.
EIGRP Load Balancing

EIGRP Equal-Cost Load Balancing
R1(config-router)# maximum–paths 3
R1(config-router)#
R1 Topology Table
Advertised Distance (AD)
Network Neighbor AD FD
172.16.2.0/24 R2 20 40
R3 20 40
R4 20 40
R5 20 40

 EIGRP can also balance traffic across multiple routes that have different metrics.
• Referred to as unequal-cost load balancing.
 The degree to which EIGRP performs load balancing is controlled with the variance
multiplier command.
• The multiplier is a value, between 1 and 128, used for load balancing.
• The default is 1, which means equal-cost load balancing.
• Setting a variance value greater than 1 allows EIGRP to install multiple loop-free routes with
unequal cost in the routing table.
• EIGRP will always install successors (the best routes) in the routing table.
• The variance allows feasible successors (and only feasible successor routes) as candidate
routes to potentially be installed in the routing table.
Unequal Cost Load Balancing

EIGRP Unequal-Cost Load Balancing
R1(config-router)# variance 2
R1(config-router)#
R1 Topology Table
Advertised Distance (AD)
Network Neighbor AD FD
172.16.2.0/24 R2 10 30
R3 10 20
R4 25 45
R5 10 50

Configuring and
Verifying EIGRP
Authentication

 Many routing protocols support authentication such that a router authenticates the source of each
routing update packet that it receives.
 Simple password authentication is supported by:
• IS-IS
• OSPF
• RIPv2
 MD5 authentication is supported by:
• OSPF
• RIPv2
• BGP
• EIGRP
Router Authentication

 Simple password authentication:
• Router sends packet and key.
• Neighbor checks if received key matches its key.
• Is not secure.
 MD5 authentication:
• Configure a “key” (password) and key-id; router generates a message digest, or hash, of the key,
key-id and message.
• Message digest is sent with packet; key is not sent.
• Is secure.
Simple Password vs. MD5 Authentication

 EIGRP supports MD5 authentication.
 Router generates and checks every EIGRP packet. Router authenticates the source of each routing
update packet that it receives.
 Configure a “key” (password) and key-id; each participating neighbor must have same key
configured.
EIGRP MD5 Authentication

 EIGRP MD5 authentication:
• Router generates a message digest, or hash, of the key, key-id, and message.
• EIGRP allows keys to be managed using key chains.
• Specify key-id (number, key, and lifetime of key).
• First valid activated key, in order of key numbers, is used.
MD5 Authentication

 The following key parameters must be defined in enough detail before configuring EIGRP
authentication:
◦ The EIGRP AS number
◦ The authentication mode (MD5)
◦ The definition of one or more keys to authenticate EIGRP packets, according to the network
security plan.
◦ The keys’ lifetime, if multiple keys are defined.
 Once defined, the following steps may be implemented:
1. Configure the authentication mode for EIGRP.
2. Configure the key chain.
3. Optionally configure the keys’ lifetime parameters.
4. Enable authentication to use the key(s) in the key chain.
Planning for EIGRP

Configure the Authentication Mode for EIGRP
 Specify MD5 authentication for EIGRP packets.
Router(config-if)#
ip authentication mode eigrp autonomous-system md5
 Enable EIGRP packet authentication using key in the key-chain.
Router(config-if)#
ip authentication key-chain eigrp autonomous-system name-
of-chain

Configure the Key Chain
 Define the keychain in key chain configuration mode.
Router(config)#
key chain name-of-chain
 Identify the key and enter the key-id configuration mode.
Router(config-keychain)#
key key-id
Router(config-keychain-key)#
key-string text
 Identify key string (password)

Configure Keys Lifetime Parameters (Optional)
 Specify when the key will be accepted for received packets.
Router(config-keychain-key)#
accept-lifetime start-time {infinite | end-time |
duration seconds}
 Specify when the key can be used for sending EIGRP
packets.Router(config-keychain-key)#
send-lifetime start-time {infinite | end-time |
duration seconds}

Enable Authentication to Use the Key Chain
 Enable EIGRP packet authentication using key in the key-chain.
Router(config-if)#
ip authentication key-chain eigrp autonomous-system
name-of-chain

Configuring EIGRP MD5 Authentication
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100R1# show running-config
!
<output omitted>
!
key chain R1chain
key 1
key-string FIRST-KEY
accept-lifetime 04:00:00 Jan 1 2009 infinite
send-lifetime 04:00:00 Jan 1 2009 04:00:00 Jan 31 2009
key 2
key-string SECOND-KEY
send-lifetime 04:00:00 Jan 25 2009 infinite
!
<output omitted>
!
interface FastEthernet0/0
ip address 172.16.1.1 255.255.255.0
!
interface Serial0/0/0
bandwidth 64
ip address 192.168.1.101 255.255.255.224
ip authentication mode eigrp 100 md5
ip authentication key-chain eigrp 100 R1chain
!
router eigrp 100
network 172.16.1.0 0.0.0.255
network 192.168.1.0
auto-summary
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102
.1.1

Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
R2# show running-config
!
<output omitted>
!
key chain R2chain
key 1
key-string FIRST-KEY
key 2
key-string SECOND-KEY
!
<output omitted>
!
interface FastEthernet0/0
ip address 172.17.2.2 255.255.255.0
!
bandwidth 64
ip address 192.168.1.102 255.255.255.224
ip authentication mode eigrp 100 md5
ip authentication key-chain eigrp 100 R2chain
!
router eigrp 100
network 172.17.2.0 0.0.0.255
network 192.168.1.0
auto-summary
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102
.1.1

Verifying MD5 Authentication
R1#
*Apr 21 16:23:30.517: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 192.168.1.102
(Serial0/0/0) is up: new adjacency
R1#
(sec) (ms) Cnt Num
0 192.168.1.102 Se0/0/0 12 00:03:10 17 2280 0 14
R1#
R1# show ip route
<output omitted>
D 172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:02:22, Serial0/0/0
R1#
R1# ping 172.17.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.17.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/15/16 ms

R1# show key chain
Key-chain R1chain:
key 1 -- text “FIRST-KEY"
accept lifetime (04:00:00 Jan 1 2009) - (always valid) [valid now]
send lifetime (04:00:00 Jan 1 2009) - (04:00:00 Jan 31 2009)
key 2 -- text “SECOND-KEY"
accept lifetime (04:00:00 Jan 25 2009) - (always valid) [valid now]
send lifetime (04:00:00 Jan 25 2009) - (always valid) [valid now]

Troubleshooting MD5 Authentication
EIGRP Packets debugging is on
(UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY, SIAREPLY)
*Jan 21 16:38:51.745: EIGRP: received packet with MD5 authentication, key id = 1
*Jan 21 16:38:51.745: EIGRP: Received HELLO on Serial0/0/0 nbr 192.168.1.102
*Jan 21 16:38:51.745: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely
0/0
R2#
*Jan 21 16:38:38.321: EIGRP: received packet with MD5 authentication, key id = 2
*Jan 21 16:38:38.321: EIGRP: Received HELLO on Serial0/0/0 nbr 192.168.1.101
*Jan 21 16:38:38.321: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely
0/0

Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
EIGRP AS 100
R1(config-if)# key chain R1chain
R1(config-keychain)# key 2
R1(config-keychain-key)# key-string wrongkey
R1(config-keychain-key)#
172.17.2.0 /24
S0/0/0
S0/0/0
64 kbps
192.168.1.96 /27
.101
.102
.1.1
*Jan 21 16:50:18.749: EIGRP: pkt key id = 2, authentication mismatch
*Jan 21 16:50:18.749: EIGRP: Serial0/0/0: ignored packet from 192.168.1.101, opcode = 5 (invalid
authentication)
*Jan 21 16:50:18.749: EIGRP: Dropping peer, invalid authentication
*Jan 21 16:50:18.749: EIGRP: Sending HELLO on Serial0/0/0
*Jan 21 16:50:18.749: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*Jan 21 16:50:18.753: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 192.168.1.101
(Serial0/0/0) is down: Auth failure
R2#
R2#

Optimizing EIGRP
Implementations

 Quantity of routing information exchanged between peers: without proper route summarization,
this can be excessive.
 Number of routers that must be involved when a topology change occurs.
 Depth of topology: the number of hops that information must travel to reach all routers.
 Number of alternate paths through the network.
Factors That Influence EIGRP Scalability

 Queries are sent when a route is lost and no feasible successor is available.
 The lost route is now in “active” state.
 Queries are sent to all neighboring routers on all interfaces except the interface to the successor.
 If the neighbors do not have their lost-route information, queries are sent to their neighbors.
 If a router has an alternate route, it answers the query; this stops the query from spreading in
that branch of the network.
EIGRP Query Process

 In a large internetwork EIGRP queries can generate many resources.
 Several solutions exist to optimize the query propagation process and to limit the amount of
unnecessary EIGRP load on the links, including:
◦ Summarization
◦ Redistribution
◦ EIGRP stub routing feature.
Overwhelming EIGRP Query Process

 If a router does not receive a reply to all the outstanding queries within default 3 minutes (180
seconds), the route goes into Stuck-in-Active (SIA) state.
 Common SIA reasons:
• A router is too busy to answer the query.
• A router cannot allocate the memory to process the query.
• The circuit between the two routers is not reliable.
• The router has unidirectional links.
 SIA solutions:
• Redesign the network to limit the query range by route summarization and the ip
summary-address eigrp command.
• Configure the remote routers as stub EIGRP routers.
Stuck-in-Active

SIA Solution: Summarization
 Poorly designed networks can make summarization difficult.
 Manually summarize the routes whenever possible to support a hierarchical network
design.
 The more networks EIGRP summarizes, the lower the number of queries being sent out.
• Ultimately reduces the occurrence of SIA errors.

SIA Solution: Summarization
 This network design is better because subnet addresses from individual major networks are
localized within each cloud, allowing summary routes configured using the ip summary-
address eigrp command to be injected into the core.
 As an added benefit, the summary routes act as a boundary for the queries generated by a
topology change.

 The EIGRP Stub Routing feature:
• Improves network stability
• Reduces resource utilization and
• Simplifies remote router (spoke) configuration
SIA Solution: Stub Networks

 Stub routing is commonly used in hub-and-spoke topology.
 Stub router sends a special peer information packet to all neighboring routers to report its status
as a stub router.
• Any neighbor that receives a packet informing it of the stub status does not query the stub
router for any routes.
• Stub routers are not queried and instead, hub routers connected to the stub router answer the
query on behalf of the stub router.
 Only the remote routers are configured as stubs.
EIGRP Stub Routing

EIGRP Stub
 Configure a router as a stub router.
eigrp stub [receive-only | connected | static | summary | redistributed]
receive-only
Restricts the router from sharing any of its routes with any other router within an
EIGRP AS.
Keyword cannot be combined with any other keyword.
connected
Permits the EIGRP stub routing feature to send connected routes.
This option is enabled by default and is the most widely practical stub option.
static
Permits the EIGRP stub routing feature to send static routes.
Redistributing static routes with the redistribute static command is still necessary.
summary
Permits the EIGRP stub routing feature to send automatically summarized and / or
manually summarized routes.
This option is enabled by default.
redistributed
Permits the EIGRP stub routing feature to send redistributed routes.
Redistributing routes with the redistribute command is still necessary.

Example: EIGRP Stub Parameters
 If stub connected is configured:
• B will advertise 10.1.2.0/24 to A.
• B will not advertise 10.1.2.0/23, 10.1.3.0/23, or
10.1.4.0/24.
 If stub summary is configured:
• B will not advertise 10.1.2.0/24, 10.1.3.0/24, or
10.1.4.0/24.

Example: EIGRP Stub Parameters (Cont.)
 If stub static is configured:
• B will not advertise 10.1.2.0/24,
10.1.2.0/23, or 10.1.3.0/24.
 If stub receive-only is configured:
• B won’t advertise anything to A, so A needs
to have a static route to the networks
behind B to reach them.

The chapter focused on the following topics:
 Features of EIGRP, including fast convergence, use of partial updates, multiple network layer
support, use of multicast and unicast, VLSM support, seamless connectivity across all data link
layer protocols and topologies, and sophisticated metric.
 EIGRP’s underlying processes and technologies—neighbor discovery/recovery mechanism, RTP,
DUAL finite state machine, and protocol-dependent modules.
 EIGRP's tables—neighbor table, topology table, and routing table
 EIGRP terminology:
• Advertised distance (the metric for an EIGRP neighbor router to reach the destination; the
metric between the next-hop router and the destination)
• Feasible distance (the sum of the AD from the next-hop neighbor, and the cost between the local
router and the next-hop router)
• Successor (a neighboring router that has a least-cost loop-free path to a destination, the lowest
FD)
• Feasible successor (a neighboring router that has a loop-free backup path to a destination).
• Passive routes, those not undergoing recomputation; active routes, those undergoing
recomputation
 The five EIGRP packet types: hello, update, query, reply, and acknowledgment.
• Updates, queries, and replies are sent reliably.
Summary

◦ EIGRP initial route discovery process, started by a router sending hello packets. Neighboring
routers reply with update packets, which populate the router's topology table. The router
chooses the successor routes and offers them to the routing table.
◦ The DUAL process including selecting FSs. To qualify as an FS, a next-hop router must have an
AD less than the FD of the current successor route for the particular network, to ensure a loop-
free network.
◦ The EIGRP metric calculation, which defaults to bandwidth (the slowest bandwidth between the
source and destination) + delay (the cumulative interface delay along the path).
◦ Planning EIGRP implementations, including:
 IP addressing
 Network topology
 EIGRP traffic engineering.
◦ The list of tasks for each router in the network include:
 Enabling the EIGRP routing protocol (with the correct AS number)
 Configuring the proper network statements
 Optionally configuring the metric to appropriate interfaces.
Summary

• Basic EIGRP configuration commands.
• Commands for verifying EIGRP operation.
• Configuring a passive-interface.
• Propagating a default route.
• EIGRP summarization.
• EIGRP over Frame Relay.
• EIGRP over MPLS.
• EIGRP load-balancing
• Configuring, verifying, and troubleshooting EIGRP MD5 authentication.
• EIGRP scalability factors, including the amount of information exchanged, the number of
routers, the depth of the topology, and the number of alternative paths through the network.
• The SIA state and how to limit the query range to help reduce SIAs.
• Configuring the remote routers as stub EIGRP routers.
• Graceful shutdown, which broadcasts a goodbye message (in a hello packet, with all K values
set to 255) when an EIGRP routing process is shut down, to inform neighbors
Summary (continued)

Understanding
OSPF Terminology
and Operation

 OSPF is a standards-based link-state IP routing protocol described in RFC 2328.
◦ It was developed to meet RIP’s inability to scale beyond 15 routers.
◦ Proposed by IETF in 1988 and formalized in 1991.
◦ There are 2 versions; OSPFv2 is for IPv4 and OSPFv3 is for IPv6.
Open Shortest Path First (OSPF)

 OSPF features include:
◦ Fast convergence
◦ Supports VLSM
◦ Efficient use of bandwidth - Routing changes trigger routing updates (no periodic updates)
◦ Supports large network size
◦ Routing based on best path selection
◦ Grouping of members into Areas
OSPF Features

 With link-state routing protocols, each router has the full picture of the network topology, and
can independently make a decision based on an accurate picture of the network topology.
 To do so, each link-state router keeps a record of:
◦ Its immediate neighbor routers.
◦ All the other routers in the network, or in its area of the network, and their attached
networks.
◦ The best paths to each destination.
Link-State Protocol Characteristics

 Respond quickly to network changes.
 Send triggered updates when a network change occurs.
 Send periodic updates (link-state refresh), at long intervals, such as every 30 minutes.
◦ Uses LSAs to confirm topology information before the information ages out of the link-state
database.
Link-State Protocol Advantages

 OSPF databases / tables:
◦ OSPF adjacency database = Neighbor table
◦ OSPF link-state database = Topology table
◦ OSPF forwarding database = Routing table
 Link-state advertisements (LSAs)
 Link-State Database (LSDB)
 Shortest-Path First (SPF) Routing Algorithm
◦ Dijkstra algorithm
 SPF Tree
 OSPF Areas
◦ Backbone (transit) and standard areas.
 Types of OSPF routers:
◦ Internal router, backbone router, Area Border Router (ABR), Autonomous System Boundary
Router (ASBR)
◦ Designated Router (DR) and Backup Designated Router (BDR)
OSPF Terminology

OSPF Router Tables / Databases
 OSPF maintains three databases which are used to create three tables.
Database Table Description
Adjacency
Database
Neighbor
Table
• List of all neighbors routers to which a router has established
bidirectional communication.
• This table is unique for each router.
• Can be viewed using the show ip ospf neighbor command.
Link-state
Database
Topology
Table
• List of information about all other routers in the network.
• The database shows the network topology.
• All routers within an area have identical link-state databases.
• Can be viewed using the show ip ospf database command.
Forwarding
Database
Routing
Table
• List of routes generated when an algorithm is run on the link-
state database.
• Each router’s routing table is unique and contains information on
how and where to send packets to other routers.
• Can be viewed using the show ip route command.

Link-State Advertisements (LSAs)
 When a change occurs in the network topology, the
router experiencing the change creates a link-state
advertisement (LSA) concerning that link.
◦ LSAs are also called link-state protocol data units
(PDUs).
 The LSA is multicasted to all neighboring devices
using either 224.0.0.5 or 224.0.0.6.
 Routers receiving the LSA immediately forward it to
all neighboring routers.

Link-State Database (LSDB)
 Routers receiving add the LSA to their link-state
database (LSDB).
 The LSDB is used to calculate the best paths
through the network.
 OSPF best route calculation is based on Edsger
Dijkstra's shortest path first (SPF) algorithm.

SPF Routing Algorithm
 The SPF algorithm accumulates costs along each
path, from source to destination.
◦ The accumulated costs is then used by the router
to build a topology table.

SPF Tree and Routing Table
 The topology table is essentially an SPF tree which
contains a listing of all OSPF networks and the costs
to reach them.
 The resulting best routes are then considered to be
added to the routing table.

 To minimize processing and memory requirements, OSPF can divide the routing topology into a
two-layer hierarchy called areas.
 Characteristics of OSPF areas include:
◦ Minimizes routing table entries.
◦ Localizes impact of a topology change within an area.
◦ Detailed LSA flooding stops at the area boundary.
◦ Requires a hierarchical network design.
OSPF Areas

OSPF Two-Layer Hierarchy
 Backbone Area
◦ Referred to as Area 0
◦ Also known as the Transit Area.
 Regular (Standard) Areas
◦ Also known as a nonbackbone areas.
◦ All regular areas must connect to the
backbone area.
◦ Standard areas can be further defined
as stub areas, totally stubby areas, and
Not-so-stubby areas (NSSAs).
 The optimal number of routers per area varies based on factors such as network stability, but Cisco
recommends:
• An area should have no more than 50 routers.
• A router should not be in more than 3 areas.

 How OSPF routers exchange information is based on:
◦ The function of the router.
◦ The type of LSAs it can forward.
◦ The type of area it resides in.
 OSPF routers may function as either:
◦ Internal router
◦ Backbone router
◦ Area Border Router (ABR)
◦ Autonomous System Boundary Router (ASBR)
 Note:
◦ A router can exist as more than one router type.
OSPF Router Types

OSPF Router Types
Internal
Router
Internal
Routers
Internal
Router
All
Backbone
Routers
ABR and
Backbone
Router
ABR and
Backbone
Router
ASBR and
Backbone
Router

 Routers that have all their interfaces within the same area.
 Internal routers in the same area:
◦ Have identical LSDBs.
◦ Run a single copy of the routing algorithm.
Internal Router

 OSPF design rules require that all areas be connected to a single backbone area (Area 0).
◦ Area 0 is also known as Area 0.0.0.0
 An Area 0 router is referred to as a backbone router.
◦ Depending on where it resides in Area 0, it may also be called an Internal router, an ABR, or an
ASBR.
Backbone Router

 Routers with interfaces attached to multiple areas and responsible for:
◦ Joining areas together.
◦ Maintaining separate link-state databases for each area.
◦ Routing traffic destined to/arriving from other areas.
◦ Summarizing information about each area connected and flooding the information through area
0 to the other areas connected.
◦ An area can have one or more ABR.
 ABR cannot send LSU’s to other areas until the entire intra-area is synchronized.
Area Border Router (ABR)

 Routers that have at least one interface connected to another AS, such as a non-OSPF network.
 Routers support redistribution.
◦ They can import non-OSPF network information to the OSPF network.
 Should reside in the backbone area.
Autonomous System Boundary Router (ASBR)

OSPF Router Types
 Routers A, B, C, D and E are backbone routers.
• Backbone routers make up Area 0.
 Routers C, D and E are area border routers (ABRs).
• ABRs attach all other areas to Area 0.
 Routers A, B, F, G, and H are internal routers.
• Internal routers are completely within an area and do not interconnect to any other area or
autonomous system (AS).

 To reduce the amount of OSPF traffic on multiaccess broadcast networks such as Ethernet, OSPF
elects:
◦ A Designated Router (DR)
◦ A Backup Designated Router (BDR)
 The DR is responsible for updating all other OSPF routers (called DROTHERs) when a change
occurs in the multiaccess network.
◦ The BDR monitors the DR and takes over should the DR fail.
 A router connected to multiple broadcast networks can be a DR on one segment and a regular
(DROTHER) router on another segment.
DR and BDR Routers

OSPF Metric Calculation
 The OSPF metric calculation is based on cost.
 Cost is an indication of the overhead required to send
packets across a certain interface.
 The cost of an interface is inversely proportional to the
bandwidth of that interface.
◦ A higher bandwidth is attributed a lower cost.
◦ A lower bandwidth is attributed a higher cost.
Bandwidth
High
Low
Lower Cost
Higher Cost

OSPF Cost Formula
 Cost = 100,000,000 / Bandwidth (bps)
 For example:
• 10BaseT = 100,000,000 / 10,000,000 = 10
• T1 = 100,000,000 / 1,544,000 = 64

 OSPF packets are used to perform several functions, including:
◦ Neighbor discovery, to form adjacencies.
◦ Flooding link-state information, to facilitate LSDBs being built in each router.
◦ Running SPF to calculate the shortest path to all known destinations.
◦ Populating the routing table with the best routes to all known destinations.
OSPF Packet

OSPF Packet
Frame Header
Frame Payload
CRC
IP
Header
Protocol Number
(OSPF = 89)
OSPF
Header
OSPF
Message
On a LAN, the OSPF
packet is encapsulated in
an Ethernet frame with a
destination multicast MAC
address of either:
• 01-00-5E-00-00-05
• 01-00-5E-00-00-06
The destination multicast IP
address is set to either:
• 224.0.0.5 (All OSPF routers
listen to this address.)
• 224.0.0.6 (All DR and BDR
routers listen to this address.
The OSPF protocol field is 89.
The OSPF header
identifies the type
of OSPF packet,
the router ID and
the area number.
The OSPF
message
contains the
packet type
specific
message
information.

OSPF Packet Types
 Five packet types make OSPF capable of sophisticated and
complex communications.
Type Packet Name Description
1 Hello
Discovers neighbors and builds
adjacencies between them.
2 DBD
Database description
Checks for database synchronization
between routers.
3 LSR
Link-state request
Requests specific link-state records from
another router.
4 LSU
Link-state update
Sends specifically requested link-state
records.
5 LSAck
Link-State Acknowledgment
Acknowledges the other packet types.

OSPF Message
Frame
Header
Frame Payload
CRC
IP
Header
Protocol
Number
(OSPF = 89)
OSPF Header OSPF MessageOSPF Message
The OSPF message contains different information, depending on the packet type:
Packet Type Contains
Type 1 - Hello Contains a list of known neighbors.
Type 2 - DBD
Contains a summary of the LSDB, which includes all known router IDs and their last sequence number,
among a number of other fields.
Type 3 - LSR Contains the type of LSU needed and the router ID of the router that has the needed LSU.
Type 4 - LSU Contains the full LSA entries. Multiple LSA entries can fit in one OSPF update packet.
Type 5 - LSAck Data field is empty.

 Hello packets are used to:
◦ Discover directly connected OSPF neighbors.
◦ Establish and maintain neighbor adjacencies with these directly connected neighbors.
◦ Advertise parameters on which two routers must agree to become neighbors.
◦ Elect the Designated Router (DR) and Backup Designated Router (BDR) on multi-access
networks like Ethernet and Frame Relay.
Type 1 - OSPF Hello Packet

 Hello packet fields must match on neighboring routers for them to establish an adjacency:
◦ Hello interval
◦ Dead interval
◦ Network type.
◦ Area id
◦ Authentication password
◦ Stub area flag
 Two routers on the same network segment may not form an OSPF adjacency if:
◦ They are not in the same area
◦ The subnet masks do not match, causing the routers to be on separate networks.
◦ The OSPF Hello or Dead Timers do not match.
◦ The OSPF network types do not match.
◦ The OSPF network command is missing or incorrect.

 By default, OSPF Hello packets are transmitted to 224.0.0.5 (all OSPF routers) every:
◦ 10 seconds (Default on multiaccess and point-to-point networks).
◦ 30 seconds (Default on NBMA networks – Frame Relay).
 The Dead interval is the period, expressed in seconds, that the router will wait to receive a Hello
packet before declaring the neighbor "down."
◦ If the Dead interval expires before the routers receive a Hello packet, OSPF will remove that
neighbor from its link-state database.
◦ The router floods the link-state information about the "down" neighbor out all OSPF enabled
interfaces.
 Cisco uses a default of 4 times the Hello interval.
◦ 40 seconds (Default on multiaccess and point-to-point networks).
◦ 120 seconds (Default on NBMA networks – Frame Relay).

 The Database Description (DBD) packets contain an abbreviated list of the sending router's link-
state database and is used by receiving routers to check against the local link-state database.
 The link-state database must be identical on all link-state routers within an area to construct an
accurate SPF tree.
Type 2 - OSPF DBD Packet

 The Link State Request (LSR) packet is used by the receiving routers to request more information
about any entry in the DBD.
Type 3 - OSPF LSR Packet

 The Link-State Update (LSU) packets are used for OSPF routing updates.
◦ They reply to LSRs as well as to announce new information.
 LSUs contain seven different types of Link-State Advertisements (LSAs).
 LSUs contains the full LSA entries.
◦ Multiple LSA entries can fit in one OSPF update packet.
Type 4 - OSPF LSU Packet

 LSAck - Link-State Acknowledgement Packet:
◦ When an LSU is received, the router sends a LSAck to confirm receipt of the LSU.
◦ The LSAck data field is empty.
Type 5 - OSPF LSAck Packet

 When an OSPF router is initially
connected to a network it
attempts to create adjacencies
with neighbors.
 To do so, it progresses through
these various states using the 5
OSPF packet types.
OSPF States
Down State
Init State
Two-Way State
ExStart State
Exchange State
Loading State
Full State
No Hello packets received = Down
Send Hello Packets
Transit to Init state
Neighbor Discovery – Hello Protocol
Hello packets received from the neighbor and it
contains the initial router’s router ID.
Transit to two-way state
(Optional) DR and BDR election
Transit to ExStart state
Database Synchronization
Negotiate master / slave relationship and DBD
packet sequence number
DBD exchanged as LSAs are requested and sent
Transit to either Loading or Full state after
completing the database description
Newly learned routes are asked for and current
database is being processed
Route Calculations
Router is synchronized with the neighbor and
route calculations using the SPF algorithm begins
More LSAs
required
Yes
No

Neighbor Discovery – Hello Protocol
R1 R2
172.16.5.0 /24
Fa0/0
.1 .2
Fa0/1
Down State
Init State 
Hello! I’m router ID 172.16.5.1. Is there anyone else on this link?
Hello
Hello! I’m router ID 172.16.5.2 and I see 172.16.5.1.
Hello
R2 neighbor list:
172.16.5.1, int Fa0/1
Unicast to R1
R1 neighbor list:
172.16.5.2, int Fa0/0
Two-Way State
Attempt State

Database Synchronization & Route Calc
R1 R2
172.16.5.0 /24
Fa0/0
.1 .2
Fa0/1
ExStart State 
I will start the exchange because I have router ID 172.16.5.1.
Hello
No, I will start the exchange because I have a higher router ID.
Hello
Here is a summary of my link-state database.
DBD

Here is a summary of my link-state database.
DBD
Exchange State

Thanks for the information!
LSAck
LSAck
Loading State

I need more information on the 172.16.6.0 network.
LSR
Here is the entry for 172.16.6.0/24.
LSU

Thanks for the information!
LSAck
Full State

 Once neighbors adjacencies have been established, the Hello packet continues to be transmitted
every 10 seconds (default) between neighbors.
◦ As long as the other routers keep receiving the Hello packets, the transmitting router and its
networks reside in the topology database.
 After the topological databases are synchronized, updates (LSUs) are sent only to neighbors when:
◦ A change is perceived (Incremental updates)
◦ Every 30 minutes (Condensed version is forwarded).
Adjacent OSPF Neighbors

 Each LSA entry has its own aging timer, which the link-state age field carries.
 The default aging timer value for OSPF is 30 minutes (1800 seconds).
 After an LSA entry ages, the router that originated the entry sends the LSA, with a higher sequence
number, in a link-state update (LSU), to verify that the link is still active.
◦ The LSU can contain one or more LSAs.
◦ This LSA validation method saves on bandwidth compared to distance-vector routers, which
send their entire routing table at short, periodic intervals.
Link-State Data Structures

OSPF Administrative Distance
Route Source Administrative Distance
Connected 0
Static 1
EIGRP Summary 5
External BGP 20
Internal EIGRP 90
IGRP 100
OSPF 110
IS IS 115
RIP 120
External EIGRP 170
Internal BGP 200

Planning OSPF
Routing
Implementations

 Prior to deploying an OSPF routing solution, the following should be considered:
◦ IP addressing plan
◦ Network topology
◦ OSPF areas
Planning to Deploy OSPF

 The information necessary to implement OSPF routing includes the following:
◦ The IP addresses to be configured on individual router interfaces.
◦ A list of routers on which OSPF is to be enabled, along with the OSPF process number to use and
the connected networks that are to run OSPF and that need to be advertised (per individual
router).
◦ The area in which each interface is to be configured.
◦ Metrics that need to be applied to specific interfaces, or OSPF traffic engineering.
 In the implementation plan, OSPF tasks include the following:
◦ Enabling the OSPF routing protocol, directly on an interface or by using the correct network
command under the OSPF routing process configuration mode.
◦ Assigning the correct area id to the interface, via the OSPF configuration on the interface or
under the OSPF routing process configuration mode.
◦ Optionally configuring the metric to appropriate interfaces.
Implementing OSPF

 After implementing OSPF, verification should confirm proper deployment on each router.
◦ Verifying that the appropriate OSPF neighbor relationships and adjacencies are established
◦ Verifying that the OSPF LSDB is populated with the necessary information.
◦ Verifying that IP routing table is populated with the necessary information.
◦ Verifying that there is connectivity in the network between routers and to other devices.
◦ Verifying that OSPF behaves as expected in a case of a topology change, by testing link failure
and router failure events.
Verifying OSPF

 After a successful OSPF deployment, the solution and verification process and results should be
◦ A topology map
◦ The IP addressing plan
◦ The area hierarchy
◦ The networks and interfaces included in OSPF on each router
◦ The default and any special metrics configured
◦ The verification results.
Documenting

Configuring and
Verifying Basic
OSPF

Enable OSPF Routing
 Define OSPF as the IP routing protocol.
Router(config)#
router ospf process-id
 The process-id is an internally used number that identifies the OSPF routing process.
 The process-id does not need to match process IDs on other routers
 It can be any positive integer in the range from 1 to 65535.

Identify OSPF Networks
 Define OSPF networks to advertise to OSPF neighbors.
network ip-address [wildcard-mask] area area-id
 The ip-address parameter can be a network, a subnet, or the address of a directly
connected interface.
 The wildcard-mask is an inverse mask used to determine how to interpret the address.
• The mask has wildcard bits, where 0 is a match and 1 is “don’t care.”
• For example, 0.0.255.255 indicates a match in the first 2 octets.
• The area-id parameter specifies the OSPF area to be associated with the address.

 Recall that a wildcard mask is the inverse of a subnet mask.
 An easy way to calculate the inverse of the subnet mask, is to subtract the subnet mask from
255.255.255.255.
 For example, the inverse of subnet mask 255.255.255.252 is 0.0.0.3.
The Wildcard Mask
255.255.255.255
– 255.255.255.252
0. 0. 0. 3

Identify OSPF Networks
 Optional method to enable OSPF explicitly on an interface.
Router(config-if)#
ip ospf process-id area area-id
 The process-id parameter can be a network, a subnet, or the address of a directly
connected interface.
• The area-id parameter specifies the OSPF area to be associated with the address.
• Because this command is configured explicitly for the interface, it takes precedence over the
network area command.

Configuring Single-Area OSPF Example
R2 R3
OSPF Area 0
S0/0/1
S0/0/1
64 kbps
10.2.1.0 /24
.2
.1
R1 Fa0/0
.1
Fa0/0
.2
10.64.0.0 /24
R2(config-if)# exit
R2(config)#
R1(config-if)# exit
R1(config)#
R3(config-if)# exit
R3(config)#

Configuring Single-Area OSPF Example
R2 R3
OSPF Area 0
S0/0/1
S0/0/1
64 kbps
10.2.1.0 /24
.2
.1
R1 Fa0/0
.1
Fa0/0
.2
10.64.0.0 /24
R1(config)# router ospf 1
R1(config-router)# network 10.0.0.0 0.255.255.255 area 0
R1(config-router)#
R2(config-router)#
R3(config-router)#

Configuring Multi-Area OSPF Example
R2 R3
OSPF Area 0
S0/0/1
S0/0/1
64 kbps
10.2.1.0 /24
.2
.1
R1 Fa0/0
.1
Fa0/0
.2
10.64.0.0 /24
OSPF Area 1
R1(config-router)#
R2(config-router)#
R3(config-router)#

Alternate Multi-Area OSPF Configuration
R2 R3
OSPF Area 0
S0/0/1
S0/0/1
64 kbps
10.2.1.0 /24
.2
.1
R1 Fa0/0
.1
Fa0/0
.2
10.64.0.0 /24
OSPF Area 1
R1(config-router)#
R2(config-if)# ip ospf 50 area 1
R2(config-if)# exit
R2(config)#
R2(config-router)#
R3(config-router)#

 A router is known to OSPF by the OSPF router ID number.
◦ LSDBs use the OSPF router ID to differentiate one router from the next.
 By default, the router ID is the highest IP address on an active interface at the moment of OSPF
process startup.
◦ However, for stability reason, it is recommended that the router-id command or a
loopback interface be configured.
OSPF Router ID

OSPF Router ID
Router ID
explicitly
configured?
Use that as
the Router-ID
Yes
No
Loopback
interface
configured?
Yes
No
Use the highest active
configured IP address
Use the highest
configured loopback IP
address

Define the Router ID
 Assign a specific router ID to the router.
router-id ip-address
 Any unique arbitrary 32-bit value in an IP address format (dotted decimal) can be used.
 If this command is used on an OSPF process that is already active, then the new router ID
takes effect:
 After the next router reload.
 After a manual restarting of the OSPF process using the clear ip ospf process
privileged EXEC command.

Verifying the Router-ID
R2 R3
OSPF Area 0
S0/0/1
S0/0/1
64 kbps
10.2.1.0 /24
.2
.1
R1 Fa0/0
.1
Fa0/0
.2
10.64.0.0 /24
OSPF Area 1
R2# show ip ospf
Routing Process “ospf 50” with ID 10.64.0.2
<output omitted>

Command Description
show ip protocols Displays OSPF process ID, router ID, networks
router is advertising & administrative distance
show ip ospf neighbors Displays OSPF neighbor relationships.
show ip route Displays the routing table.
show ip ospf interface Displays hello interval and dead interval
show ip ospf
Displays OSPF process ID, router ID, OSPF area
information & the last time SPF algorithm
calculated
Verifying OSPF

Verifying OSPF: show ip protocols
Routing Protocol is “ospf 1”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 10.64.0.1
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
10.0.0.0 0.255.255.255 area 0
Reference bandwidth unit is 100 mbps
<output omitted>

Verifying OSPF: show ip ospf neighbors
R2# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.64.0.1 1 FULL/DROTHER 00:00:30 10.64.0.1 FastEthernet0/0
10.2.1.1 1 FULL/ - 00:00:34 10.2.1.1 Serial0/0/1
Display OSPF neighbors.
Lists the neighbors
in the order they
were learned.
The OSPF priority
of the interface.
The OSPF state of the interface.
FULL state means that the router
and its neighbor have identical
OSPF link-state databases.
The amount of time remaining that
the router will wait to receive an
OSPF Hello packet from the
neighbor before declaring the
neighbor down.
The IP address of the neighbor's
interface to which this router is
directly connected.
The interface on which
this router has formed
adjacency with the
neighbor.

Verifying OSPF: show ip route ospf
R1# show ip route ospf
O IA 10.2.1.0/24 [110/782] via 10.64.0.2, 00:03:05, FastEthernet0/0
R1#
Verify that the router recognizes OSPF routes.

 To clear all routes from the IP routing table, use:
Router# clear ip route *
 To clear a specific route from the IP routing table, use:
Router# clear ip route A.B.C.D
Clearing the OSPF Routing Table

Verifying OSPF: show ip ospf interface
R1# show ip ospf interface fastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 10.64.0.1/24, Area 0
Process ID 1, Router ID 10.64.0.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DROTHER, Priority 0
Designated Router (ID) 10.64.0.2, Interface address 10.64.0.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:04
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 4
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 10.64.0.2 (Designated Router)
Suppress hello for 0 neighbor(s)
Verify OSPF configured interfaces.

Verifying OSPF: show ip ospf
R2# show ip ospf
Routing Process “ospf 50” with ID 10.64.0.2
<output omitted>
Area BACKBONE(0)
Area has no authentication
SPF algorithm last executed 00:01:25.028 ago
SPF algorithm executed 7 times
<output omitted>
Area 1
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm last executed 00:00:54.636 ago
SPF algorithm executed 3 times
<output omitted>
R2#
Verify general OSPF information.

Understanding OSPF
Network Types

 OSPF defines three types of network:
OSPF Network Types
Network Type Description Example
Broadcast
• A multiaccess broadcast network.
• DR / BDR required.
All Ethernet networks
Point-to-point
• A network that joins a single pair of routers.
• No DR / BDR required.
Serial link using PPP /
HDLC
Nonbroadcast
multiaccess
(NBMA)
• A network that interconnects more than two routers but
that has no broadcast capability.
• DR / BDR may or may not be required.
• There are five modes of OSPF operation available for
NBMA networks:
• RFC-compliant modes:
• non-broadcast
• point-to-multipoint
• Cisco proprietary modes:
• broadcast
• point-to-multipoint non-broadcast
• point-to-point
• The choice of mode depends on the topology of the
NBMA network.
Frame Relay
ATM
X.25

Broadcast
 DR /BDR election required since there could be many devices.
• Establishing adjacencies with all routers in a broadcast network would easily overload a router
due to the overhead of maintaining those adjacencies.
• Instead, OSPF routers form full adjacencies with the DR and BDR only.
 Packets to all OSPF routers are forwarded to 224.0.0.5.
 Packets to the DR / BDR are forwarded to 224.0.0.6.

Broadcast Challenge: Multiple Adjacencies
 A challenge of broadcast network is the number of adjacencies that would be required.
• One adjacency for every pair of routers.
• This would increase network traffic and load on each router to manage each individual adjacency.

Broadcast Challenge: Extensive LSAs
 Another challenge is the increase in network LSAs.
• Every LSA sent out also requires an acknowledgement.
 Consequence:
• Lots of bandwidth consumed
• Chaotic traffic

Solution: Designated Router
 A designated router (DR) and backup designated router (BDR) solve these challenges because they:
• Reduce routing update traffic
• Manage link-state synchronization

 The DR is elected and becomes responsible for maintaining the topology table for the segment.
 This DR has two main functions:
◦ To become adjacent to all other routers on the network segment.
◦ To act as a spokesperson for the network.
 As spokesperson the DR becomes the focal point for collecting and sending routing information
(LSAs).
Designated Router (DR)

 For fault tolerance, a second router is elected as the BDR.
◦ The BDR must also become adjacent to all routers on the network and must serve as a second
focal point for LSAs.
◦ However, the BDR is not responsible for updating the other routers or sending network LSAs.
 The BDR keeps a timer on the DR's update activity to ensure that it is operational.
◦ If the BDR does not detect activity from the DR after the timer expires, the BDR immediately
becomes the DR and a new BDR is elected.
Backup Designated Router (BDR)

 DRs and BDRs are elected on a per-network basis and therefore each network segment has its own
DR and BDR.
◦ For example, a router connected to multiple multiaccess broadcast networks can be a DR on one
segment and a regular (DROTHER) router on another segment.
 The election process is accomplished dynamically using the Hello protocol.
◦ However, the election can be manually manipulated the ip ospf priority number
interface configuration command.
 After a DR and BDR have been selected, any router added to the broadcast network establishes full
adjacencies with the DR and BDR only.
DR/BDR

Assigning Router Priority
 Assign a specific OSPF priority to the router.
Router(config-if)#
ip ospf priority number
 A router interface can have a priority number between 0 - 255:
 0 = DROTHER - Router cannot be a DR
 1 = Favorable - Default for all routers
 255 = Very favorable- Ensures at least of a tie.
 The priority must be configured before the election takes place to figure into the election.
 To display an interface's priority value and other key information use the show ip ospf interface
command.

1. All neighbors with a priority > 0 are listed.
2. The router with highest priority is elected BDR.
If there is a tie, the highest router IDs are used.
3. If there is no DR, the BDR is promoted as DR.
4. The neighbor with the next highest priority is elected BDR.
The Election of the DR

 The DR / BDR maintain these roles until they fail even when more routers with higher priorities
show up on the network.
 To influence the election of DR & BDR, do one of the following:
◦ Boot up the DR first, followed by the BDR, and then boot all other routers.
OR
◦ Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR,
and then all other routers.
Manipulating the Election Process

Point-to-Point
 Both routers become fully adjacent to each another.
 Usually a serial interface running either PPP or HDLC.
• May also be a point-to-point subinterface running Frame Relay or ATM.
 No DR /BDR election required since there are only two devices.
 OSPF autodetects this type of network.
 Packets are sent to 224.0.0.5.

 Multi-Protocol Label Switching (MPLS) is an Internet Engineering Task Force (IETF) standard
architecture that combines the advantages of Layer 3 routing with the benefits of Layer 2
switching.
 A unique feature of MPLS is its capability to perform label stacking, in which multiple labels can
be carried in a packet.
 The top label, which is the last one in, is always processed first.
◦ Label stacking enables multiple LSPs to be aggregated, thereby creating tunnels through
multiple levels of an MPLS network.
OSPF over MPLS

OSPF over Layer 3 MPLS VPN
 The customer and provider edge routers are running OSPF.
• However the internal provider routers do not.
 The customer has to agree upon OSPF parameters with the service provider (SP) to ensure
connectivity.
• These parameters are often governed by the SP.

OSPF over Layer 2 MPLS VPN
 The Layer 2 MPLS VPN backbone and the provider routers are not visible to the customer routers.
• A neighbor relationship is established directly between OSPF enabled routers over the MPLS
backbone, and behaves in the same way as on an Ethernet broadcast network therefore DR and
BDR routers are elected.

 Frame Relay, ATM, and X.25 are examples of NBMA networks.
 The default OSPF hello and dead intervals on NBMA interfaces are 30 seconds and 120 seconds,
respectively.
 Although NBMA networks can support more than two routers, they have no inherent broadcast
capability.
◦ This can create reachability issues.
 To implement broadcasting or multicasting, the router replicates the packets to be broadcast or
multicast and sends them individually on each permanent virtual circuit (PVC) to all destinations.
◦ This process is CPU and bandwidth intensive.
Nonbroadcast Multiaccess (NBMA)

 By default, OSPF cannot automatically build adjacencies with neighbor routers over NBMA
interfaces.
 OSPF considers the NBMA environment to function similarly to other multiaccess media such as
Ethernet.
◦ However, NBMA networks are usually hub-and-spoke (star) topologies using PVCs or
switched virtual circuits (SVCs).
◦ In these cases, the physical topology does not provide the multiaccess capability on which
OSPF relies.
 The election of the DR becomes an issue in NBMA topologies because the DR and BDR need to
have full Layer 2 connectivity with all routers in the NBMA network.
 The DR and BDR also need to have a list of all the other routers so that they can establish
adjacencies.
DR Election in an NBMA Topology

OSPF over NBMA Topology
 Depending on the network topology, several OSPF configuration choices are available for a Frame
Relay network.

 There are five NBMA topology modes of operation:
◦ Two official OSPF modes described in RFCs
◦ Three customized Cisco modes.
 RFC 2328-compliant modes are as follows:
◦ Nonbroadcast (NBMA)
◦ Point-to-multipoint
 Cisco modes are as follows:
◦ Point-to-multipoint nonbroadcast
◦ Broadcast
◦ Point-to-point
 OSPF NBMA topology modes are configured using the ip ospf network interface
◦ Some modes require that a neighbor be manually configured using the neighbor router
OSPF over NBMA Topology

Assign an NBMA Topology Mode
 Define an OSPF network type on an interface.
Router(config-if)#
ip ospf network [{non-broadcast | point-to-multipoint [non-
broadcast] | broadcast | point-to-point}]
 The choice of mode depends on the NBMA topology.
 The default OSPF mode on a Frame Relay:
◦ Interface is non-broadcast mode.
◦ Point-to-point subinterface is point-to-point mode.
◦ Multipoint subinterface is non-broadcast mode.

NBMA Modes Description
non-broadcast
(RFC-compliant)
• One IP subnet.
• Neighbors must be manually configured.
• DR and BDR are elected.
• DR and BDR need to have full connectivity with all other routers.
• Typically used in a full- or partial-mesh topology.
point-to-multipoint
(RFC-compliant)
• One IP subnet.
• Uses a multicast OSPF hello packet to automatically discover the neighbors.
• DR and BDR are not required. The router sends additional LSAs with more information about
neighboring routers.
• Typically used in a partial-mesh or star topology.
point-to-multipoint
nonbroadcast
(Cisco proprietary)
• If multicast and broadcast are not enabled on the VCs, the RFC-compliant point-to-multipoint
mode cannot be used, because the router cannot dynamically discover its neighboring routers
using the hello multicast packets; this Cisco mode should be used instead.
• Neighbors must be manually configured.
• DR and BDR election is not required.
broadcast
(Cisco proprietary)
• Makes the WAN interface appear to be a LAN.
• One IP subnet.
• Uses a multicast OSPF hello packet to automatically discover the neighbors.
• DR and BDR are elected.
• Full- or partial-mesh topology.
point-to-point
(Cisco proprietary)
• Different IP subnet on each subinterface.
• No DR or BDR election.
• Used when only two routers need to form an adjacency on a pair of interfaces.
• Interfaces can be either LAN or WAN.
NBMA Topology Modes of Operation

Identify a Neighboring Router
 Statically define adjacent relationships in NBMA networks.
neighbor ip-address [priority number] [poll-interval
number] [cost number] [database-filter all]
ip-address • Specifies the IP address of the neighboring router.
priority number
• (Optional) Specifies priority of neighbor. The default is 0, which
means that the neighboring router does not become the DR or BDR.
poll-interval number
• (Optional) Specifies how long an NBMA interface waits before
sending hellos to the neighbors even if the neighbor is inactive. The
poll interval is defined in seconds.
cost number
• (Optional) Assigns a cost to the neighbor in the form of an integer
from 1 to 65535. Neighbors with no specific cost configured assume
the cost of the interface based on the ip ospf cost command.
• For point-to-multipoint interfaces, the cost keyword and the
number argument are the only options that are applicable. This
keyword does not apply to nonbroadcast mode.
database-filter all • (Optional) Filters outgoing LSAs to an OSPF neighbor.

Non-Broadcast Mode Example (Full-Mesh)
 Characteristics of the RFC-compliant non-
broadcast parameter include:
◦ A full-mesh topology is typically used
therefore the DR and BDR are
dynamically elected.
 DR / BDR require full connectivity
with all other routers.
◦ One IP subnet.
◦ OSPF neighbors must be manually
configured.
R1(config-if)# ip ospf network non-broadcast
R1(config-if)# exit
R1(config-router)# neighbor 192.168.1.2
R1(config-router)# neighbor 192.168.1.3

Non-Broadcast Mode Example (Partial-Mesh)
 Characteristics of the RFC-
compliant non-broadcast
parameter include:
◦ If a partial-mesh topology is used
then the DR and BDR are elected
manually using the priority
parameter on the hub router.
◦ One IP subnet.
configured.
R1(config-if)# ip ospf network non-broadcast
R1(config-if)# exit
R1(config-router)# neighbor 192.168.1.2 priority 0

Point-to-multipoint Mode Example
 Characteristics of the RFC-
compliant point-to-
multipoint parameter
include:
◦ Used with partial-mesh or hub-
and-spoke (star) topology.
◦ One IP subnet.
◦ DR and BDR not required.
◦ Uses multicast OSPF hello packets
to dynamically discover
neighbors.
R1(config-if)# ip ospf network point-to-multipoint
R1(config-if)# exit
R1(config-router)#

Point-to-multipoint non-broadcast Mode
 Characteristics of Cisco’s
point-to-multipoint
non-broadcast parameter
include:
configured.
◦ Used in special cases where
neighbors cannot be
automatically discovered.
R1(config-if)# ip ospf network point-to-multipoint non-broadcast
R1(config-if)# exit
R1(config-router)# neighbor 192.168.1.2 cost 10
R1(config-router)# neighbor 192.168.1.3 cost 20

Broadcast Mode Example
broadcast parameter include:
◦ DR and BDR are elected and
require full connectivity with all
other routers.
◦ Can be configured for a full-mesh
topology or a static election of the
DR based on the interface
priority.
◦ One IP subnet.
◦ Uses multicast OSPF hello packets
to dynamically discover
neighbors.
R1(config-if)# ip ospf network broadcast
R1(config-if)# exit
R1(config-router)#

Point-to-point Mode Example
point-to-point parameter
include:
◦ Partial mesh or star topology.
◦ Only IP subnet.
R1(config-if)# encapsulation frame-relay
R1(config-if)# ip ospf network point-to-point
R1(config-if)# exit
R1(config-router)#

 OSPF can also be run over subinterfaces.
◦ A subinterface is a physical interface that can be split into multiple logical interfaces.
◦ Each subinterface requires an IP subnet.
 Subinterfaces can be defined as either a point-to-point or multipoint interface.
◦ A point-to-point subinterface has similar properties to a physical point-to-point interface.
 Note:
 The ip ospf network command is not required.
Subinterfaces

Define a Subinterface
 Define a subinterface.
Router(config)#
interface serial number.subinterface-number {multipoint
| point-to-point}
number.subinterface-
number
Specifies the interface number and subinterface number.
The subinterface number is in the range of 1 to
4294967293.
The interface number that precedes the period (.) is the
interface number to which this subinterface belongs.
multipoint
Specifies that the subinterface is multipoint; on multipoint
subinterfaces routing IP, all routers are in the same subnet.
point-to-point
Specifies that the subinterface is point-to-point; on point-to-
point subinterfaces routing IP, each pair of point-to-point
routers is in its own subnet.

Using Point-to-point Subinterfaces
 Characteristics:
◦ Same properties as any physical
point-to-point physical interface
◦ One IP subnet per subinterface
pair.
◦ Used when only 2 routers need to
form an adjacency on a pair of
interfaces.}
R1(config-if)# interface S0/0/0.1 point-to-point
R1(config-subif)# ip address 10.1.1.1 255.255.255.0
R1(config-subif)# interface S0/0/0.2 point-to-point
R1(config-subif)# router ospf 1

Using Multipoint Subinterfaces
 The example has one point-to-point
subinterface and one multipoint
subinterface.
◦ The multipoint subinterface supports two
other routers in a single
 Multipoint Frame Relay subinterfaces
default to OSPF nonbroadcast mode,
which requires neighbors to be
statically configured and a DR and
BDR election.
R1(config-if)# interface S0/0/0.1 point-to-point
R1(config-subif)# interface S0/0/0.2 multipoint
R1(config-subif)# router ospf 1

OSPF over NBMA Topology Summary
OSPF Mode
NBMA
Preferred
Topology
Subnet
Address Hello Timer Adjacency RFC or
Cisco Example
Non-broadcast
Full or partial
mesh
Same 30 sec
Manual
configuration
DR/BDR elected
RFC
Frame Relay
configured on a serial
interface
Point-to-
multipoint
Partial mesh or
star
Same 30 sec
Automatic
No DR/BDR
RFC
OSPF over Frame
Relay mode that
eliminates the need
for a DR; used when
VCs support multicast
and broadcast
Point-to-
multipoint
nonbroadcast
Partial mesh or
star
Same 30 sec
Manual
configuration
No DR/BDR
Cisco
OSPF over Frame
Relay mode that
eliminates the need
for a DR; used when
VCs do not support
multicast and
broadcast
Broadcast
Full or partial
mesh
Same 10 sec
Automatic
DR/BDR elected
Cisco
LAN interface such
as Ethernet
Point-to-point
Partial mesh or
star, using
subinterfaces
Different for
each
subinterface
10 sec
Automatic
No DR/BDR
Cisco
Serial interface with
point-to-point
subinterfaces

 LSAs are the building blocks of the OSPF LSDB.
◦ Individually, LSAs act as database records.
◦ When combined, they describe the entire topology of an OSPF area.
 There are several types of OSPF network LSAs
◦ Not all are in use.
LSAs
LSA Type Description
1 Router LSA
2 Network LSA
3 and 4 Summary LSAs
5 AS external LSA
6 Multicast OSPF LSA
7 Defined for NSSAs
8 External attributes LSA for Border Gateway Protocol (BGP)
9, 10, or 11 Opaque LSAs

LSA Type 1: Router LSA
 Generated by all routers in an area to describe their directly attached links (Intra-area routes).
• Floods within its area only and cannot cross an ABR.
• LSA includes list of directly attached links and is identified by the router ID of the originating
router
• Routing Table Entry = O

LSA Type 1: Link Types
Link Type Description Link-state ID
1
Point-to-point connection to another
router
Neighboring router ID
2 Connection to a transit network IP address of DR
3 Connection to a stub network IP network/subnet number
4 Virtual link Neighboring router ID

LSA Type 2: Network LSA
 Advertised by the DR of the broadcast network.
• Floods within its area only; does not cross ABR.
• Link-state ID is the DR.
• Routing Table Entry = O

LSA Type 3: Summary LSA
 Advertised by the ABR of originating area.
• Regenerated by subsequent ABRs to flood throughout the autonomous system.
• By default, routes are not summarized, and type 3 LSA is advertised for every subnet.
• Link-state ID is the network or subnet advertised in the summary LSA
• Routing Table Entry = O IA

LSA Type 4: Summary LSA
 Generated by the ABR of the originating area to advertise an ASBR to all other areas in the
autonomous system.
• They are regenerated by all subsequent ABRs to flood throughout the autonomous system.
• Link-state ID is the router ID of the ASBR.
• Routing Table Entry = O IA

LSA Type 5: External LSA
 Used by the ASBR to advertise networks from other autonomous systems.
• Type 5 LSAs are advertised and owned by the originating ASBR.
• The Link-state ID is the external network number.
• Routing Table Entry = O E1 or O E2

LSA Type 7: NSSA LSA
 Generated by an ASBR inside a Not-so-stubby area (NSSA) to describe routes redistributed into
the NSSA.
• LSA 7 is translated into LSA 5 as it leaves the NSSA.
• Routing Table Entry = O N1 or O N2
• Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost up to the
ASBR.
ABR

Interpreting the
OSPF LSDB and
Routing Table

Interpreting the OSPF Database
R1# show ip ospf database
OSPF Router with ID (10.0.0.11) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.0.11 10.0.0.11 548 0x80000002 0x00401A 1
10.0.0.12 10.0.0.12 549 0x80000004 0x003A1B 1
100.100.100.100 100.100.100.100 548 0x800002D7 0x00EEA9 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.31.1.3 100.100.100.100 549 0x80000001 0x004EC9
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.0.0 10.0.0.11 654 0x80000001 0x00FB11
10.1.0.0 10.0.0.12 601 0x80000001 0x00F516
<output omitted>
Use the show ip ospf database command to gather link state information.

 Each LSA in the LSDB maintains a sequence number.
◦ The sequence numbering scheme is a 4-byte number that begins with 0x80000001 and ends
with 0x7FFFFFFF.
 OSPF floods each LSA every 30 minutes to maintain proper database synchronization.
◦ Each time the LSA is flooded, the sequence number is incremented by one.
 Ultimately, an LSA sequence number will wrap around to 0x80000001.
◦ When this occurs, the existing LSA is prematurely aged to maxage (one hour) and flushed.
 When a router encounters two instances of an LSA, it must determine which is more recent.
◦ The LSA having the newer (higher) LS sequence number is more recent.
LSA Sequence Numbering

Route Designator in Routing Table
R1# show ip route
<output omitted>
O IA 172.31.2.0 [110/1563] via 10.1.1.1, 00:12:35, FastEthernet0/0
O IA 172.31.1.0 [110/782] via 10.1.1.1, 00:12:35, FastEthernet0/0
C 10.200.200.13/32 is directly connected, Loopback0
O 10.1.2.0/24 [110/782] via 10.1.3.4, 00:12:35, Serial0/0/0
O 10.1.0.0/24 [110/782] via 10.1.1.1, 00:12:37, FastEthernet0/0
O E2 10.254.0.0/24 [110/50] via 10.1.1.1, 00:12:37, FastEthernet0/0
Route Designator Description
O OSPF intra-area (router LSA) and network LSA
• Networks from within the router’s area.
Advertised by way of router LSAs and
network LSAs.
O IA OSPF interarea (summary LSA)
• Networks from outside the router’s
area but within the OSPF AS.
Advertised by way of summary LSAs.
O E1 Type 1 external routes
• Networks from outside the router’s AS,
advertised by way of external LSAs.
O E2 Type 2 external routes
• Networks from outside the router’s AS,
advertised by way of external LSAs.

1. All routers calculate the best paths to destinations within their area (intra-area) and add these
entries to the routing table.
◦ Includes type 1 and 2 LSAs, noted with a designator of O.
2. All routers calculate the best paths to the other areas.
◦ Includes type 3 and 4 LSAs, noted with a designator of O IA.
3. All routers (except stub areas) calculate the best paths to the external autonomous system (type
5) destinations.
◦ Includes either external type 1 (E1), indicated with an O E1 or external type 2 (E2), indicated
with an O E2.
Best Path Calculation

 The cost of an external route varies, depending on the external type configured on the ASBR.
 An ASBR can be configured to send out two types of external routes into OSPF.
◦ Denoted in the routing table as E1 for Type 1
◦ Denoted in the routing table as E2 for Type 2.
 Depending on the type, OSPF calculates the cost of external routes differently.
ASBR – Type 1 and 2 Routes

 O E1 Routes
◦ The metric is calculated by adding the external cost to the internal cost of each link that the
packet crosses.
 Use this packet type when there are multiple ASBRs advertising a route to the same
autonomous system.
 O E2 Routes
◦ The packet will always have the external cost assigned, no matter where in the area it crosses.
 Default setting on ASBRs.
 Use this packet type if only one router is advertising a route to the autonomous system.
 Type 2 routes are preferred over Type 1 routes unless two equal cost routes exist to the
destination.
ASBR – Type 1 and 2 Routes

E2 Routes
 By default, RTA uses a Type 2 metrics to send external routing information.
 RTB will receive the external RIP routes, including 9.0.0.0/8 from RTA.
 When RTB forwards this route, the metric for the external route remains the same (in this case, 20).

E1 Routes
 If RTA is configured to use a Type 1 metric with external routes, OSPF will increment the metric
value of the external route according to its standard cost algorithm.

Configuring OSPF LSDB Overload Protection
 Limit the processing of LSAs for a defined OSPF process.
max-lsa maximum-number [threshold-percentage] [warning-only]
[ignore-time minutes] [ignore-count count-number] [reset-
time minutes]
maximum-number Maximum number of LSAs that the OSPF process can keep in the OSPF LSDB.
threshold-percentage
(Optional) The percentage of the maximum LSA number, as specified by the maximum-
number argument, at which a warning message is logged. The default is 75 percent.
warning-only
(Optional) Specifies that only a warning message is sent when the maximum limit for
LSAs is exceeded; the OSPF process never enters ignore state. Disabled by default.
ignore-time minutes
(Optional) Specifies the time, in minutes, to ignore all neighbors after the maximum limit
of LSAs has been exceeded. The default is 5 minutes.
ignore-count count-number
(Optional) Specifies the number of times that the OSPF process can consecutively be
placed into the ignore state. The default is five times.
reset-time minutes
(Optional) Specifies the time, in minutes, after which the ignore count is reset to 0. The
default is 10 minutes.

Configuring and
Verifying
Advanced OSPF
Features

OSPF Passive-Interface
 Prevent OSPF updates out a specified router interface.
passive-interface type number [default]
 Set a particular interface or all router interfaces to passive.
 The default option sets all router interfaces to passive.
 For OSPF, the command:
 The specified interface appears as a stub network in the OSPF domain
 The OSPF routing information is neither sent nor received through the specified router
interface.
 Prevents neighbor relationships from being established.

Passive-Interface Example
R1(config-router)#
R2(config-router)#
Alternate configuration:
Fa0/0Fa0/0
R1 R2
172.16.1.0 /24
Internet
192.168.1.0 /27
172.17.2.0 /24
64 kbps
192.168.1.96 /27
.101
.102
S0/0/1
S0/0/0
S0/0/0
.1
.1.1

 To propagate a default route in OSPF, use the default-information originate router
◦ A default static rote also needs to be configured on the originating router
 Once configured, the default route has to be propagated into the OSPF domain.
Propagating a Default Route

default-information originate Command
 Configures a router to generate a default external route into an OSPF routing domain.
default-information originate [always] [metric metric-
value] [metric-type type-value] [route-map map-name]
always
(Optional) Specifies that OSPF always advertises the default route regardless of
whether the router has a default route in the routing table.
metric
metric-value
(Optional) A metric used for generating the default route. If you omit a value and do
not specify a value using the default-metric router configuration command, the
default metric value is 1. Cisco IOS Software documentation indicates that the
default metric value is 10; testing shows that it is actually 1.
metric-type
type-value
(Optional) The external link type that is associated with the default route that is
advertised into the OSPF routing domain. It can be one of the following values: 1—
Type 1 external route 2—Type 2 external route. The default is type 2 external route
(indicated by O*E2 in the routing table).
route-map
map-name
(Optional) Specifies that the routing process generates the default route if the route
map is satisfied.

default-information originate Example
R1(config-router)# default-information originate metric 10
R1(config-router)# exit
R1(config)# ip route 0.0.0.0 0.0.0.0 172.16.1.2
R1(config)#
R1
R2
OSPF Domain
ISP A.1
172.16.1.0 /24
.2
ISP B
10.1.1.1
10.2.1.1
.1
172.17.1.0 /24
.2
0.0.0.0 Cost 10
0.0.0.0 Cost 100
R2(config-router)# default-information originate metric 100
R2(config)# ip route 0.0.0.0 0.0.0.0 172.17.1.2
R2(config)#

 Route summarization involves consolidating multiple routes into a single advertisement.
 Proper route summarization directly affects the bandwidth, memory and CPU, that are consumed
by the OSPF process.
◦ If a network link fails or flaps, the topology change will not be propagated into the backbone or
other areas.
◦ It protects routers from needless routing table recalculations.
◦ Because the SPF calculation places a significant demand on the router's CPU, proper
summarization is an imperative part of OSPF configuration.
Route Summarization

Using Route Summarization
IA 172.16.16.0 255.255.252.0
IA 172.16.8.0 255.255.248.0

 Inter-area summarization
◦ Performed at the ABR and creates Type 3 LSAs.
 External summarization
◦ Performed at the ASBR and creates Type 5 LSAs.
 Both have the same fundamental requirement of contiguous addressing.
 If summarization is not configured correctly and there are multiple ASBRs, or multiple ABRs in
an area, suboptimal routing is possible.
◦ For example, summarizing overlapping ranges from two different routers can cause packets
to be sent to the wrong destination.
Types of Route Summarization

Intra-Area Summarization
 Configure an ABR to summarize routes for a specific area.
area area-id range address mask [advertise | not-
advertise] [cost cost]
area area-id Identifies the area subject to route summarization.
address The summary address designated for a range of addresses.
mask The IP subnet mask used for the summary route.
advertise
(Optional) Sets the address range status to advertise and generates a type
3 summary LSA.
not-advertise
(Optional) Sets the address range status to DoNotAdvertise. The type 3
summary LSA is suppressed, and the component networks remain hidden
from other networks.
cost cost
(Optional) Metric or cost for this summary route, which is used during the
OSPF SPF calculation to determine the shortest paths to the destination.
The value can be 0 to 16777215.

Intra-Area Summarization Example
R1(config-router)# area 0 range 172.16.96.0 255.255.224.0
R1(config-router)#
R2(config-router)#
Area 0
172.16.96.0 /24 - 172.16.127.0 /24
Area 1
172.16.32.0 /24 - 172.16.63.0 /24
Area 2
172.16.64.0 /24 - 172.16.95.0 /24
R1 R2

External Summarization
 Configure an ASBR to summarize external routes.
summary-address ip-address mask [not-advertise] [tag tag]
ip-address The summary address designated for a range of addresses.
mask The IP subnet mask used for the summary route.
not-advertise (Optional) Used to suppress routes that match the address/mask pair.
tag tag
(Optional) A tag value that can be used as a “match” value to control
redistribution via route maps.

External Summarization
R1(config-router)# summary-address 172.16.32.0 255.255.224.0
R1(config-router)#
External AS – RIPv2
172.16.32.0 /24 – 172.16.63.0 /24
OSPF Area 1
172.16.64.0 /24
OSPF Area 0
R1 R2
ABR
.1

Virtual Links
 Virtual links are used to connect a discontiguous area to area 0.
 A logical connection is built between router A and router B.
 Virtual links are recommended for backup or temporary connections.

LSAs on Virtual Links
 LSAs usually age out after 30 minutes.
• However, LSAs learned across virtual links have the DoNotAge (DNA) option set.
• Required to prevent excessive flooding over virtual links.
 To identify an area as a virtual link, use the area area-id virtual-link router

Configuring Virtual Links
 Define an OSPF virtual link.
area area-id virtual-link router-id [authentication
[message-digest | null]] [hello-interval seconds]
[retransmit-interval seconds] [transmit-delay
seconds] [dead-interval seconds] [[authentication-
key key] | [message-digest-key key-id md5 key]]
area-id Specifies the area ID of the transit area for the virtual link.
router-id Specifies the router ID of the virtual link neighbor.
authentication (Optional) Specifies an authentication type.
message-digest (Optional) Specifies the use of MD5 authentication.
null (Optional) Overrides authentication if configured.
hello-interval seconds (Optional) Specifies the time between the hello packets (default 10).
retransmit-interval seconds (Optional) Specifies the time between LSA retransmissions (default 5).
transmit-delay seconds (Optional) Specifies the time to send an LSU packet (default 1).
dead-interval seconds (Optional) Specifies the dead-interval time (default 40).
authentication-key key (Optional) Specifies the password for simple password authentication.
message-digest-key key-id md5 key (Optional) Identifies the key ID and key for MD5 authentication.

Virtual-Link Example
R1(config-router)# area 1 virtual-link 10.2.2.2
R1(config-router)#
Area 0
10.0.0.0
Area 1
172.16.0.0
Area 0
10.0.0.0
R1 R2
Router-ID
10.1.1.1
Router-ID
10.2.2.2
R2(config-router)# area 1 virtual-link 10.1.1.1
R2(config-router)#

Verifying a Virtual-Link Example
Area 0
10.0.0.0
Area 1
172.16.0.0
Area 0
10.0.0.0
R1 R2
Router-ID
10.1.1.1
Router-ID
10.2.2.2
R1# show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 10.2.2.2 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 1, via interface Serial0/0/1, Cost of using 781
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:07
Adjacency State FULL (Hello suppressed)
Index 1/2, retransmission queue length 0, number of retransmission 1
First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec
R1#

Changing the Reference Bandwidth
 The reference bandwidth defaults to 108 (100,000,000 bps or 100 Mbps).
• This can be a problem when using interfaces faster than 100 Mbps and higher since they
would all have the same OSPF cost of 1.
 The reference bandwidth can be modified using the auto-cost reference-
bandwidth router configuration command.

Changing the Reference Bandwidth
 Change the reference bandwidth for faster interfaces.
auto-cost reference-bandwidth ref-bw
 The rf-bw parameter is the reference bandwidth in megabits per
second.
 The range is from 1 to 4,294,967.
 The default is 100.
 Use this command if interfaces are faster than 100 Mbps.
 The command must be configured on all OSPF routers to ensure accurate
route calculations.

 The cost of a link can be modified using either the:
◦ bandwidth interface command
◦ ip ospf cost interface command
 The configured bandwidth value is used by the SPF algorithm to calculate the cost.
◦ For example, configuring the bandwidth 128 command on a serial interface would generate
a cost of 1,562.
 Cost = 100,000,000 / 128,000 = 1,562.
 Using the ip ospf cost interface command achieves the same result without the calculation.
◦ For example, the interface cost could be statically configured using the ip ospf cost 1562
command.
Modifying the Cost of a Link

Override the Default Interface Cost
 Manually define the cost of an interface.
Router(config-if)#
ip ospf cost interface-cost
 The interface-cost is an integer from 1 to 65,535.
 The lower the number, the better (and more preferred) the link.
 Can be used as an alternative to the bandwidth command.

OSPF Two-Layer Hierarchy - Review
 Backbone Area
• Referred to as Area 0
• Also known as the Transit Area.
 Regular (Standard) Areas
• Also known as a nonbackbone areas.
• All regular areas must connect to the backbone area.

OSPF Special Area Types
 The OSPF standard area can be further divided into four types of stub areas:
• Stub area
• Totally stubby area
• NSSA
• Totally stubby NSSA

OSPF Area Types
Area Type
Accepts routes
within area
(O)
Accepts routes
from other
areas
(O IA)
Accepts
external routes
(O E1 and O E2)
Allows ASBR
Cisco
proprietary
Standard Yes Yes Yes Yes No
Backbone Yes Yes Yes Yes No
Stub Yes Yes
No
(uses default route)
No No
Totally stubby Yes
No
No
No Yes
NSSA Yes Yes
No
Yes No
Totally stubby
NSSA
Yes
No
No
Yes Yes

 An area qualifies as stub or totally stubby area if it has the following characteristics:
◦ The area is not the backbone area (area 0).
◦ There is a single exit point from that area.
 If there are multiple exits, one or more ABRs should inject a default route into the stub area
however suboptimal routing paths might occur.
◦ There is no ASBR inside the area.
◦ The area is not used as a transit area for virtual links.
Stub and Totally Stub Area Characteristics

 All OSPF routers inside the stub area, including ABRs, are configured as stub routers using the
area area-id stub router configuration command.
 By default, the ABR of a stubby or totally stubby area advertises a default route with a cost of 1.
◦ To change the cost of the default route, use the area area-id default-cost cost
router configuration command.
Stub and Totally Stub Area Characteristics

Configure a Stub Area
 Identify an area as a stub network.
area area-id stub
 The area-id parameter is the identifier for the stub area and can be
either a decimal value or a value in dotted-decimal format, like an IP
address.

Change the Default Cost
 Define the cost of the default route injected into the stub stubby area.
area area-id default-cost cost
 The cost parameter is for the default summary route.
 The acceptable values are 0 through 16777215.
 The default is 1.
 If this command has not been configured, the ABR will advertise 0.0.0.0
with a default cost metric of 1 plus any internal costs.

Stub Area
 Typically used in a hub-and-spoke network.
 Area does not accept external summary routes from non-OSPF sources (e.g., RIP, EIGRP).
• Specifically, it does not accept Types 4 and 5 LSAs.
• A default route (0.0.0.0) is propagated throughout the area to send a packet to an external
network.

Configuring a Stub Area
R3(config)# interface FastEthernet0/0
R3(config-if)# interface Serial 0/0/0
R3(config-if)# router ospf 100
R3(config-router)# network 192.168.14.0.0 0.0.0.255 area 0
R3(config-router)# area 2 stub
R3(config-router)#
External AS
R3
ABR
.1
R4
.1 .2
Fa0/0 S0/0/0 S0/0/0
192.168.15.0 /30192.168.14.0 /24
OSPF Area 0 Stub Area 2
R4(config-router)#

Totally Stubby Area
 Cisco proprietary solution that is better than stub area.
 Area does not accept external AS routes or inter-area routes.
• Specifically, it does not accept Types 3, 4 and 5 LSAs.
• It recognizes only intra-area routes and the default route 0.0.0.0.
• A default route (0.0.0.0) is propagated throughout the area.

Configure a Totally Stubby Area
 Identify an ABR as a totally stubby network.
area area-id stub no-summary
 Command is only configured on the ABR.
 All other routers in the totally stubby area are configured as stub routers.
 The area-id parameter is the identifier for the stub area and can be either a decimal
value or a value in dotted-decimal format, like an IP address.
 The no-summary parameter stops summary LSAs, in addition to external LSAs, from
flooding into the totally stubby area.

Configuring a Totally Stubby Area
R3(config)# interface FastEthernet0/0
R3(config-router)# area 2 stub no-summary
R3(config-router)#
External AS
R3
ABR
.1
R4
.1 .2
Fa0/0 S0/0/0 S0/0/0
192.168.15.0 /30192.168.14.0 /24
OSPF Area 0 Totally Stubby Area 2
R4(config-router)#

Not-So-Stubby Area (NSSA)
 Similar to a Stub Area, except that it is primarily used to connect to ISPs, or when redistribution is
required.
• Specifically, it does not accept Types 4 and 5 LSAs.
• Allows the importing of external routes as Type 7 LSAs and converts them to Type 5 LSAs on the
ABR.
• Better than creating stub areas and also useful for spokes.
RIP AS
172.16.20.0 /24
R2
ABR
R3
192.168.15.0 /30
NSSA Area 1 Area 0
R1
RIP Type 7 LSA : 172.16.20.0/24 Type 5 LSA : 172.16.0.0/16

Configure an NSSA
 Define an NSSA area.
area area-id nssa no-redistribution] [default-information-
originate] [metric metric-value] [metric-type type-value] [no-
summary]
area-id The identifier for the NSSA.
no-redistribution
(Optional) Used when the router is an NSSA ABR and you want the
redistribute command to import routes only into the standard areas, but not
into the NSSA area.
default-information-
originate
(Optional) Used to generate a type 7 default LSA into the NSSA area. This
keyword takes effect only on an NSSA ABR or an NSSA ASBR.
metric metric-value
(Optional) Metric that is used for generating the default route. Acceptable
values are 0 through 16777214.
metric-type type-
value
(Optional) OSPF metric type for default routes. It can be one of the following
values: type 1 external route or 2: type 2 external route
no-summary
(Optional) Allows an area to be a totally stubby NSSA, which is like an NSSA
but does not have summary routes injected into it.

Configuring a NSSA Area
R1(config-router)# redistribute rip subnets
R1(config-router)# default metric 150
R1(config-router)# area 1 nssa
R1(config-router)#
R2(config-router)# area 1 nssa default-information-originate
R2(config-router)#
RIP AS
172.16.10.0
172.16.11.0 R2
ABR
.2
Fa0/0
172.17.0.0172.17.20.0 /24
NSSA Area 1 Area 0
R1
.1
Fa0/0
0.0.0.0 Default Route

 Cisco proprietary solution to NSSA.
 Area does not accept external AS routes or inter-area routes.
◦ Specifically, it does not accept Types 3, 4 and 5 LSAs.
◦ It recognizes only intra-area routes and the default route 0.0.0.0.
◦ A default route (0.0.0.0) is propagated throughout the area.
 The ABR of a totally stubby NSSA must be configured with the no-summary keyword to prevent
the flooding of summary routes for other areas into the NSSA area.
Totally Stubby NSSA

Configuring a Totally Stubby NSSA Area
R1(config-router)# redistribute rip subnets
R1(config-router)# area 1 nssa
R1(config-router)#
R2(config-router)# area 1 nssa no-summary
R2(config-router)#
RIP AS
172.16.10.0
172.16.11.0 R2
ABR
.2
Fa0/0
172.17.0.0172.17.20.0 /24
NSSA Area 1 Area 0
R1
.1
Fa0/0
0.0.0.0 Default Route

 Depends on the type of area.
 In a standard area:
◦ Routers do not automatically generate default routes.
◦ The default-information originate command must be used.
 In a stub and totally stubby area:
◦ The ABR automatically generates a summary LSA with the link-state ID 0.0.0.0.
◦ The default-information originate command is not required.
◦ This is true even if the ABR does not have a default route.
 In an NSSA area:
◦ The ABR generates the default route, but not by default.
◦ To force the ABR to generate the default route, use the area area-id nssa default-
information-originate command.
 In a totally stubby NSSA:
◦ The ABR automatically generates a default route.
How Does OSPF Generate Default Routes?

Example OSPF Area Types in a Network

Configuring and
Verifying
Advanced OSPF
Authentication

OSPF Authentication
 Purpose is to authenticate routing information.
• This is an interface specific configuration.
• Routers will only accept routing information from other routers that have been configured with
the same authentication information.

 Router generates and checks each packet and authenticates the source of each update packet it
receives
 Requires a pre-defined “key” (password)
◦ Note: All participating neighbors must have the same key configured
 OSPF supports 2 types of authentication:
◦ Simple password authentication (plain text)
 Less secure
◦ MD5 authentication
 More secure and recommended
OSPF Authentication Types

 The following key parameters must be defined in enough detail before configuring OSPF
authentication:
◦ The authentication mode (simple password versus MD5)
◦ The definition of one or more keys to authenticate OSPF packets, according to the network
security plan.
 Once defined, the following steps may be implemented:
1. Assign a password (key) to be used.
 The actual command varies depending on the authentication mode used.
2. Specify the authentication mode (simple password or MD5).
Planning for OSPF

Configure A Key for Simple Authentication
 Define a password to use for simple password authentication.
Router(config-if)#
ip ospf authentication-key password
 The password parameter can be entered up to 8 bytes in length.
 This command is used in conjunction with the ip ospf
authentication command.

Configure the MD5 Key-ID and Key
 Define a password to use for MD5 authentication.
Router(config-if)#
ip ospf message-digest-key key-id md5 key
 The key-id parameter is an identifier in the range from 1 to 255.
 The key parameter can be entered up to 16 bytes in length.
 All neighboring routers on the same network must have the same
key-id and the same key value.
 This command is used in conjunction with the ip ospf
authentication message-digest command.

Configure the Authentication Mode for OSPF
 Specify the authentication type.
Router(config-if)#
ip ospf authentication [message-digest | null]
 Before using this command, configure a password.
 The command without any parameters specifies that simple password
authentication will be used.
 The message-digest parameter specifies that MD5 authentication
will be used.
 The null parameter specifies that no authentication is used.
 This can be useful for overriding simple password or MD5
authentication.

Configuring Simple Password Authentication
!
<output omitted>
!
interface Fa0/0
ip address 10.1.1.1 255.255.255.0
!
<output omitted>
!
ip address 192.168.1.101 255.255.255.224
ip ospf authentication
ip ospf authentication-key PLAINPAS
!
<output omitted>
!
router ospf 10
log-adjacency-changes
network 10.1.1.1 0.0.0.0 area 0
network 192.168.1.0 0.0.0.255 area 0
!
<output omitted>
Fa0/0Fa0/0
R1 R2
10.1.1.0 /24
Area 0
10.2.2.0 /24
S0/0/1
S0/0/1
192.168.1.96 /27
.101
.102
.1.1

Configuring Simple Password Authentication
!
<output omitted>
!
interface Fa0/0
ip address 10.2.2.1 255.255.255.0
!
<output omitted>
!
ip address 192.168.1.102 255.255.255.224
ip ospf authentication
ip ospf authentication-key PLAINPAS
!
<output omitted>
!
router ospf 10
network 10.2.2.1 0.0.0.0 area 0
network 192.168.1.0 0.0.0.255 area 0
!
<output omitted>
Fa0/0Fa0/0
R1 R2
10.1.1.0 /24
Area 0
10.2.2.0 /24
S0/0/1
S0/0/1
192.168.1.96 /27
.101
.102
.1.1

Verifying Simple Password Authentication
R1# debug ip ospf adj
OSPF adjacency events debugging is on
R1#
<output omitted>
*Feb 17 18:42:01.250: OSPF: 2 Way Communication to 10.2.2.1 on Serial0/0/1,
state 2WAY
*Feb 17 18:42:01.250: OSPF: Send DBD to 10.2.2.1 on Serial0/0/1 seq 0x9B6 opt
0x52 flag 0x7 len 32
*Feb 17 18:42:01.262: OSPF: Rcv DBD from 10.2.2.1 on Serial0/0/1 seq 0x23ED
opt0x52 flag 0x7 len 32 mtu 1500 state EXSTART
*Feb 17 18:42:01.262: OSPF: NBR Negotiation Done. We are the SLAVE
*Feb 17 18:42:01.262: OSPF: Send DBD to 10.2.2.1 on Serial0/0/1 seq 0x23ED opt
<output omitted>
10.2.2.1 0 FULL/ - 00:00:34 192.168.1.102 Serial0/0/1
Displays adjacency-related events of a successful connection.

Troubleshooting Simple Password Problems
 Simple authentication on R1, no authentication on R2:
R1#
*Feb 17 18:51:31.242: OSPF: Rcv pkt from 192.168.1.102, Serial0/0/1
: Mismatch Authentication type. Input packet specified type 0, we
use type 1
R2#
: Mismatch Authentication type. Input packet specified type 1, we
use type 0

Troubleshooting Simple Password Problems
 Simple authentication on R1 and R2, but different passwords.
R1#
: Mismatch Authentication Key - Clear Text
R2#
: Mismatch Authentication Key - Clear Text

Configuring MD5 Authentication
!
<output omitted>
!
interface Fa0/0
ip address 10.1.1.1 255.255.255.0
!
<output omitted>
!
ip address 192.168.1.101 255.255.255.224
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 SECRETPASS
!
<output omitted>
!
router ospf 10
network 10.1.1.1 0.0.0.0 area 0
network 192.168.1.0 0.0.0.255 area 0
!
<output omitted>
Fa0/0Fa0/0
R1 R2
10.1.1.0 /24
Area 0
10.2.2.0 /24
S0/0/1
S0/0/1
192.168.1.96 /27
.101
.102
.1.1

!
<output omitted>
!
interface Fa0/0
ip address 10.2.2.1 255.255.255.0
!
<output omitted>
!
ip address 192.168.1.102 255.255.255.224
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 SECRETPASS
!
<output omitted>
!
router ospf 10
network 10.2.2.1 0.0.0.0 area 0
network 192.168.1.0 0.0.0.255 area 0
!
<output omitted>
Fa0/0Fa0/0
R1 R2
10.1.1.0 /24
Area 0
10.2.2.0 /24
S0/0/1
S0/0/1
192.168.1.96 /27
.101
.102
.1.1

R1# show ip ospf interface
Serial0/0/1 is up, line protocol is up
Internet Address 192.168.1.101/27, Area 0
Process ID 10, Router ID 10.1.1.1, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT
<output omitted>
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 10.2.2.1
Suppress hello for 0 neighbor(s)
Message digest authentication enabled
Youngest key id is 1
<output omitted>
R1#
10.2.2.1 0 FULL/ - 00:00:31 192.168.1.102 Serial0/0/1
R1#

R1# debug ip ospf adj
OSPF adjacency events debugging is on
<output omitted>
*Feb 17 17:14:06.530: OSPF: Send with youngest Key 1
*Feb 17 17:14:06.546: OSPF: 2 Way Communication to 10.2.2.2 on Serial0/0/1,
state 2WAY
*Feb 17 17:14:06.546: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0xB37 opt
*Feb 17 17:14:06.562: OSPF: Rcv DBD from 10.2.2.2 on Serial0/0/1 seq 0x32F
opt 0x52 flag 0x7 len 32 mtu 1500 state EXSTART
*Feb 17 17:14:06.562: OSPF: NBR Negotiation Done. We are the SLAVE
*Feb 17 17:14:06.562: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0x32F opt
<output omitted>
10.2.2.2 0 FULL/ - 00:00:35 192.168.1.102 Serial0/0/1

Troubleshooting MD5 Authentication
R1#
*Feb 17 17:56:26.502: OSPF: Rcv pkt from 192.168.1.102, Serial0/0/1 : Mismatch
Authentication Key - No message digest key 2 on interface
R2#
*Feb 17 17:55:28.286: OSPF: Rcv pkt from 192.168.1.101, Serial0/0/1 : Mismatch
Authentication Key - No message digest key 1 on interface
MD5 authentication on both R1 and R2, but R1 has key 1 and R2 has key 2,
both with the same passwords:

 Characteristics of link-state routing protocols.
 OSPF's two-tier hierarchical area structure, with a backbone area 0 and regular areas.
 How OSPF routers use the Hello protocol to build adjacencies.
 The OSPF metric calculation, which is based on the link bandwidth.
 The five types of OSPF packets—hello, DBD, LSR, LSU, and LSAck.
 The neighbor states that OSPF interfaces may pass through: down, init, two-way, exstart, exchange, loading, and
full.
 The five fields in the hello packet must match on neighboring routers: hello interval, dead interval, area id,
authentication password, and stub area flag.
 Planning OSPF implementations, including the IP addressing, network topology, and OSPF areas.
 Basic OSPF configuration commands including:
◦ router ospf process-id global configuration command
◦ network ip-address wildcard-mask area area-id interface configuration command
◦ ip ospf process-id area area-id [secondaries none] interface configuration command
◦ bandwidth kilobits interface configuration command
◦ router-id ip-address router configuration command
Summary

 Commands for verifying OSPF operation:
◦ show ip protocols
◦ show ip ospf neighbor
◦ show ip route
◦ show ip route ospf
◦ show ip ospf interface
◦ show ip ospf
◦ debug ip ospf events
◦ debug ip ospf adj
◦ debug ip ospf packet
 How the OSPF router ID is selected with the router-id ip-address router configuration command, the
highest IP address on any active loopback interface, or the highest IP address of any active physical interface
when OSPF starts.
 The three types of networks defined by OSPF: point-to-point, broadcast, and NBMA.
 How a DR and BDR are selected.
 The different types of OSPF routers: internal routers, backbone routers, ABRs, and ASBRs.
 The 11 different OSPF LSA types.
 The three kinds of OSPF routes: intra-area (O), interarea (O IA), and external (either O E1 or O E2).
 Configuring OSPF LSDB overload protection using the max-lsa router configuration command.
Summary (cont.)

 Using the passive-interface type number [default] router configuration command.
 Propagate an OSPF default route using the default-information originate [always] router
 OSPF summarization can be configured on an ABR using the area area-id range address mask
[advertise | not-advertise] [cost cost] router configuration command, and on an ASBR using
the summary-address ip-address mask [not-advertise] [tag tag] router configuration
command.
 Virtual links are configured with the area area-id virtual-link router-id router configuration
command, and verified with the show ip ospf virtual-links command.
 The several area types defined in OSPF: standard areas, backbone (transit) areas, stub areas, totally stubby
areas, NSSAs, and totally stubby NSSAs.
 The types of OSPF authentication: null, simple password authentication (also called plain-text authentication),
and MD5 authentication.
 The commands to configure OSPF simple password authentication:
◦ ip ospf authentication-key password interface configuration command
◦ ip ospf authentication interface configuration command or the area area-id
authentication router configuration command
 The commands to configure OSPF MD5 authentication:
◦ ip ospf message-digest-key key-id md5 key interface configuration command
◦ ip ospf authentication message-digest interface configuration command or the area
area-id authentication message-digest router configuration command
Summary (cont.)

BGP Terminology,
Concepts, and
Operation

IGP versus EGP
 Interior gateway protocol (IGP)
• A routing protocol operating within an Autonomous System (AS).
• RIP, OSPF, and EIGRP are IGPs.
 Exterior gateway protocol (EGP)
• A routing protocol operating between different AS.
• BGP is an interdomain routing protocol (IDRP) and is an EGP.

 An AS is a group of routers that share similar routing policies and operate within a single
administrative domain.
 An AS typically belongs to one organization.
◦ A single or multiple interior gateway protocols (IGP) may be used within the AS.
◦ In either case, the outside world views the entire AS as a single entity.
 If an AS connects to the public Internet using an exterior gateway protocol such as BGP, then it must
be assigned a unique AS number which is managed by the Internet Assigned Numbers Authority
(IANA).
Autonomous Systems (AS)

IANA
 The IANA is responsible for allocating AS numbers through five Regional Internet Registries (RIRs).
• RIRs are nonprofit corporations established for the purpose of administration and registration of
IP address space and AS numbers in key geographic locations.

Regional Internet Registries (RIRs)
RIR Name Geographic Coverage Link
AfriNIC Continent of Africa www.afrinic.net
APNIC
(Asia Pacific Network
Information Centre)
Asia Pacific region www.apnic.net
ARIN
(American Registry for Internet
Numbers)
Canada, the United States,
and several islands in the
Caribbean Sea and North
Atlantic Ocean
www.arin.net
LACNIC
(Latin America and Caribbean
Internet Addresses Registry)
Central and South America
and portions of the Caribbean
www.lacnic.net
RIPE
(Réseaux IP Européens)
Europe, the Middle East, and
Central Asia
www.ripe.net

 AS numbers can be between 1 to 65,535.
◦ RIRs manage the AS numbers between 1 and 64,512.
◦ The 64,512 - 65,535 numbers are reserved for private use (similar to IP Private addresses).
◦ The IANA is enforcing a policy whereby organizations that connect to a single provider use an
AS number from the private pool.
 Note:
◦ The current AS pool of addresses is predicted to run out by 2012.
◦ For this reason, the IETF has released RFC 4893 and RFC 5398.
◦ These RFCs describe BGP extensions to increase the AS number from the two-octet (16-bit) field
to a four-octet (32-bits) field, increasing the pool size from 65,536 to 4,294,967,296 values.
AS Numbers

 The Internet is a collection of autonomous systems that are interconnected to allow
communication among them.
◦ BGP provides the routing between these autonomous systems.
 BGP is a path vector protocol.
 It is the only routing protocol to use TCP.
◦ OSPF and EIGRP operate directly over IP. IS-IS is at the network layer.
◦ RIP uses the User Datagram Protocol (UDP) for its transport layer.
 BGP version 4 (BGP-4) is the latest version of BGP.
◦ Defined in RFC 4271.
◦ Supports supernetting, CIDR and VLSM .
 BGP4 and CIDR prevent the Internet routing table from becoming too large.
◦ Without CIDR, the Internet would have 2,000,000 + entries.
◦ With CIDR, Internet core routers manage around 300,000 entries.
◦ http://bgp.potaroo.net/
BGP Basics

# of Current BGP Routes
7 Day BGP Profile: 24-August-2010 00:00 - 30-August-2010 23:59 (UTC+1000)
Number of BGP Update Messages: 1195261
Number of Prefix Updates: 2787149
Number of Prefix Withdrawals: 490070
Average Prefixes per BGP Update: 2.74
Average BGP Update Messages per second: 1.73
Average Prefix Updates per second: 4.74
Peak BGP Update Message Rate per second: 3848 (19:25:51 Mon, 30-Aug-2010)
Peak Prefix Update Rate per second: 66398 (07:07:37 Mon, 30-Aug-2010)
Peak Prefix Withdraw Rate per second: 16512 (19:26:14 Mon, 30-Aug-2010)
Prefix Count: 342962
Updated Prefix Count: 332145
Stable Prefix Count: 10817
Origin AS Count: 35292
Updated Origin AS Count: 34786
Stable Origin AS Count: 506
Unique Path Count: 215660
Updated Path Count: 195814
Stable Path Count: 19846
As of August 30, 2010, there were 332,145 routes in the routing tables of the
Internet core routers.
http://bgpupdates.potaroo.net/instability/bgpupd.html

 A “BGP peer,” also known as a “BGP neighbor,” is a specific term that is used for BGP speakers that
have established a neighbor relationship.
 Any two routers that have formed a TCP connection to exchange BGP routing information are called
BGP peers or BGP neighbors.
Peers = Neighbors

BGP Operational Overview
 When two routers establish a TCP enabled BGP connection, they are called neighbors or peers.
• Peer routers exchange multiple connection messages.
 Each router running BGP is called a BGP speaker.

BGP Operational Overview
 When BGP neighbors first establish a connection, they exchange all candidate BGP routes.
• After this initial exchange, incremental updates are sent as network information changes.

BGP Use Between AS
 BGP provides an interdomain routing system that guarantees the loop-free exchange of routing
information between autonomous systems.

 BGP works differently than IGPs because it does not make routing decisions based on best path
metrics.
◦ Instead, BGP is a policy-based routing protocol that allows an AS to control traffic flow using
multiple BGP attributes.
 Routers running BGP exchange network attributes including a list of the full path of BGP AS
numbers that a router should take to reach a destination network.
 BGP allows an organization to fully use all of its bandwidth by manipulating these path attributes.
Comparison BGP with IGPs

Comparing IGPs with BGP
Protocol
Interior or
Exterior
Type
Hierarchy
Required?
Metric
RIP Interior
Distance
vector
No Hop count
OSPF Interior Link state Yes Cost
IS-IS Interior Link state Yes Metric
EIGRP Interior
Advanced
distance
vector
No Composite
BGP Exterior Path vector No
Path vectors
(attributes)

 Modern corporate IP networks connect to the global Internet.
 Requirements that must be determined for connecting an enterprise to an ISP include the
following:
◦ Public IP address space
◦ Enterprise-to-ISP connection link type and bandwidth
◦ Connection redundancy
◦ Routing protocol
Connecting Enterprise Networks to an ISP

 Public IP addresses are used:
◦ By internal enterprise clients to access the Internet using NAT.
◦ To make enterprise servers accessible from the Internet using static NAT.
 Public IP addresses are available from ISPs and RIRs.
◦ Most enterprises acquire their IP addresses and AS number from ISPs.
◦ Large enterprises may want to acquire IP addresses and AS number from a RIR.
Public IP Address Space

 Which connection options does the ISP offer?
 Which routing options does the ISP offer?
 Will the enterprise network be connected to multiple ISPs?
 Does the routing need to support one link to an ISP or multiple links, to one or multiple ISPs?
 Is traffic load balancing over multiple links required?
 How much routing information needs to be exchanged with the ISP?
 Does the routing need to respond to the changes in the network topology, such as when a link
goes down?
Connection and Routing Questions

Using Static Routes Example
 Static routes are the simplest way to implement routing with an ISP.
• Typically a customer has a single connection to an ISP and the customer uses a default route
toward the ISP while the ISP deploys static routes toward the customer.
PE(config)# ip route 10.0.0.0 255.0.0.0 serial 0/0/1
Company A
Internet
PER1 S0/0/1
10.0.0.0
172.16.0.0
172.17.0.0
ISP
S0/0/0
R1(config)# ip default-network 0.0.0.0
R1(config)# ip route 0.0.0.0 0.0.0.0 serial 0/0/0

Using Layer 2 Circuit Emulation Example
 Service providers may offer Layer 2 MPLS VPN to connect Company A’s sites.
• The VPN provides a Layer 2 service across the backbone and Company A’s edge routers are
connected together on the same IP subnet.
• There is no routing exchange between the ISP and Company A.

Using Layer 3 MPLS VPN Example
 Service providers may offer Layer 3 MPLS VPN.
• The VPN provides a Layer 3 service across the backbone and Company A’s edge routers are
connected to ISP edge routers using different IP subnets.
• Routing between the customer and ISP is required.

 BGP can be used to dynamically exchange routing information.
 BGP can also be configured to react to topology changes beyond a customer-to-ISP link.
Using BGP
Company A
AS 65010
Internet
PER1 S0/0/1
ISP
AS 65020
S0/0/0

 Redundancy can be achieved by deploying redundant links, deploying redundant devices, and
using redundant components within a router.
◦ The ISP connection can also be made redundant.
 When a customer is connected to a single ISP the connection is referred to as single-homed or
dual-homed.
 When a customer is connected to multiple ISPs the connection is referred to as multihomed or
dual-multihomed.
Connection Redundancy

Connection Redundancy
Dual-multihomedDual-homed
MultihomedSingle-homed
Connecting to Two or more ISPsConnecting to One ISP

Connecting to One ISP: Single-Homed
 The connection type depends on the ISP offering (e.g., leased line, xDSL, Ethernet) and link failure
results in a no Internet connectivity.
 The figure displays two options:
• Option 1: Static routes are typically used with a static default route from the customer to the ISP,
and static routes from the ISP toward customer networks.
• Option 2: When BGP is used, the customer dynamically advertises its public networks and the ISP
propagates a default route to the customer.
Company A
AS 65010
Internet
PER1 S0/0/1
ISP
AS 65020
S0/0/0
Static Route(s)Default Route
BGP
Option 1:
Option 2:

Connecting to One ISP: Dual-Homed
 The figure displays two dual-homed options:
• Option 1: Both links can be connected to one customer router.
• Option 2: To enhance resiliency, the two links can terminate at separate routers in the customer’s
network.
Company A
Internet
ISP
Option 1:
PER1
Company A
Internet
ISP
Option 2:
PER1
R2

Connecting to One ISP: Dual-Homed
 Routing deployment options include:
• Primary and backup link functionality in case the primary link fails.
• Load sharing using Cisco Express Forwarding (CEF).
 Regardless, routing can be either static or dynamic (BGP).
Company A
Internet
ISP
Option 1:
PER1
Company A
Internet
ISP
Option 2:
PER1
R2

Connecting to Multiple ISPs: Multihomed
 Connections from different ISPs can terminate on the same router, or on different routers to
further enhance the resiliency.
 Routing must be capable of reacting to dynamic changes therefore BGP is typically used.
Company A
Internet
ISP 1
PE
R1
R2
ISP 2
PE

Connecting to Multiple ISPs: Multihomed
 Multihomed benefits include:
• Achieving an ISP-independent solution.
• Scalability of the solution, beyond two ISPs.
• Resistance to a failure to a single ISP.
• Load sharing for different destination networks between ISPs.
Company A
Internet
ISP 1
PE
R1
R2
ISP 2
PE

Connecting Multiple ISPs: Dual-Multihomed
 Dual multihomed includes all the benefits of multihomed connectivity, with enhanced resiliency.
 The configuration typically has multiple edge routers, one per ISP, and uses BGP.
Company A
Internet
ISP 1
PE
R1
R2
ISP 2
PE

 When BGP is running between routers in different AS, it is called External BGP (EBGP).
 When BGP is running between routers in the same AS, it is called Internal BGP (IBGP).
Using BGP in an Enterprise Network
EBGP
EBGP
IBGP
IBGP
IBGP

 EBGP neighbors are in different autonomous systems.
◦ EBGP neighbors need to be directly connected.
External BGP

 Define neighbors:
◦ A TCP session (three-way handshake) must be established before starting BGP routing update
exchanges.
 Reachability:
◦ EBGP neighbors are usually directly connected.
 Different AS number:
◦ EBGP neighbors must have different AS numbers.
EBGP Neighbor Relationship Requirements

 IBGP neighbors are in the same autonomous systems.
◦ IBGP neighbors do not need to be directly connected.
Internal BGP

 Define neighbors:
◦ A TCP session (three-way handshake) must be established before starting BGP routing update
exchanges.
 Reachability:
◦ IBGP neighbors must be reachable usually by using an IGP.
◦ Loopback IP addresses are typically used to identify IBGP neighbors.
 Same AS number:
◦ IBGP neighbors must have the same AS number.
IBGP Neighbor Relationship Requirements

IBGP in a Transit AS
 A transit AS is an AS that routes traffic from one external AS to another external AS.
 In this example, AS 65102 is a service provider network.
• Only the two edge routers (router B and E) are running BGP and have established an IBGP
neighbor relationship using OSPF.
• Although the EBGP routes could be redistributed into OSPF, the potential number of BGP routes
may overwhelm OSPF and is therefore not recommended.

IBGP in a Transit AS
 A better solution for a provider network would be to have a fully meshed BGP internetwork.
• BGP runs on all internal routers and all routers establish IBGP sessions.
• IBGP routers have complete knowledge of external routes.

 A nontransit AS is an AS that does not route traffic from one external AS to another external AS.
◦ Nontransit AS networks are typically enterprise networks.
 All routers in a nontransit AS must still have complete knowledge of external routes.
 To avoid routing loops within an AS, BGP specifies that routes learned through IBGP are never
propagated to other IBGP peers.
◦ It is assumed that the sending IBGP neighbor is fully meshed with all other IBGP speakers and
has sent each IBGP neighbor the update.
IBGP in a Nontransit AS

BGP in an Enterprise Example
 Enterprise AS 65500 is learning routes from
both ISP-A and ISP-B via EBGP and is also
running IBGP on all of its routers.
◦ If one of the connections to the ISPs goes
down, traffic will be sent through the other
ISP.
 An undesirable situation could occur if the
enterprise AS is configured as a transit AS.
◦ For example, AS 65500 learns the
172.18.0.0/16 route from ISP-A.
◦ If router B advertises that route to ISP-B,
then ISP-B may decide to use it.
◦ This undesirable configuration could be
avoided through careful BGP
configuration.

1. Each ISP passes only a default route to the AS.
◦ The default route is passed on to internal routers.
2. Each ISP passes only a default route and provider-owned specific routes to the AS.
◦ These routes may be propagated to internal routers, or all internal routers in the transit path
can run BGP to exchange these routes.
3. Each ISP passes all routes to the AS.
◦ All internal routers in the transit path run BGP to exchange these routes.
Three Multihoming Connection Options

Default Routes from All Providers

Default Routes and Partial Updates

Full Routes from All Providers

 Internal routing protocols announce a list of networks and the metrics to get to each network.
 In contrast, BGP routers exchange network reachability information, called path vectors, made up
of path attributes.
 The path vector information includes:
◦ A list of the full path of BGP AS numbers (hop by hop) necessary to reach a destination network.
◦ Other attributes including the IP address to get to the next AS (the next-hop attribute) and how
the networks at the end of the path were introduced into BGP (the origin code attribute).
BGP Path Vector Characteristics

 Most appropriate when the effects of BGP are well-understood and at least one of the following
conditions exists:
◦ The AS has multiple connections to other autonomous systems.
◦ The AS allows packets to transit through it to reach other autonomous systems (eg, it is a service
provider).
◦ Routing policy and route selection for traffic entering and leaving the AS must be manipulated.
When to Use BGP

 Do not use BGP if one or more of the following conditions exist:
◦ A single connection to the Internet or another AS.
◦ Lack of memory or processor power on edge routers to handle constant BGP updates.
◦ You have a limited understanding of route filtering and the BGP path-selection process.
 In these cases, use static or default routes instead.
When Not to Use BGP

 The BGP synchronization rule states that:
◦ “A BGP router should not use, or advertise to an external neighbor, a route learned by IBGP,
unless that route is local or is learned from the IGP.”
◦ If synchronization is enabled, a router learning a route via IBGP waits until the IGP has
propagated the route within the autonomous system and then advertises it to external peers.
◦ With the default of synchronization disabled, BGP can use and advertise to external BGP
neighbors routes learned from an IBGP neighbor that are not present in the local routing
table.
 BGP synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later.
◦ It was on by default in earlier Cisco IOS Software releases.
BGP Synchronization

 BGP keeps its own table for storing BGP information received from and sent to BGP neighbors.
◦ This table is also known as the BGP table, BGP topology table, BGP topology database, BGP
routing table, and the BGP forwarding database.
 The router offers the best routes from the BGP table to the IP routing table.
 Neighbor table
◦ List of BGP neighbors
 BGP table (forwarding database)
◦ List of all networks learned from each neighbor
◦ Can contain multiple paths to destination networks
◦ Contains BGP attributes for each path
 IP routing table
◦ List of best paths to destination networks
BGP Table

 There are four different BGP message types:
BGP Message Types
16 2 1 1 2 2 4 1 7
Marker Length Type Version AS Hold Time BGP ID
Optional
Length
Optional
Open Message
Octets
16 2 1 2 Variable 2 Variable Variable
Marker Length Type
Unfeasible Routes
length
Withdrawn
Routes
Attribute
Length
Attributes NLRI
Update Message
Octets
16 2 1 1 1 Variable
Marker Length Type Error Code
Error
Sub-code
Diagnostic
Data
Notification Message
Octets
16 2 1
Marker Length Type
Keepalive Message
Octets

 All messages begin with the same 3 field headers
BGP Message Header
16 2 1 1 2 2 4 1 7
Optional
Length
Optional
Open Message
Octets
Marker Length Type
Unfeasible Routes
length
Withdrawn
Routes
Attribute
Length
Attributes NLRI
Update Message
Octets
16 2 1 1 1 Variable
Error
Sub-code
Diagnostic
Data
Octets
16 2 1
Marker Length Type
Keepalive Message
Octets

 Once a TCP connection has been established, the Open message is sent and includes a set of
parameters that have to be agreed upon before a full BGP adjacency can be established.
 Once both BGP peers have agreed upon mutual capabilities, they can start exchanging routing
information by means of BGP Update messages.
Open Message
16 2 1 1 2 2 4 1 7
Optional
Length
Optional
Open Message
Octets

 Update messages contain all the information BGP uses to construct a loop-free picture of the
internetwork.
 A BGP update message has information on one path only; multiple paths require multiple update
messages.
◦ All the attributes in the update message refer to that path, and the networks are those that can
be reached through it.
 An update message includes the following information:
◦ Unreachable routes information
◦ Path attribute information
◦ Network-layer reachability information (NLRI)
 This field contains a list of IP address prefixes that are reachable by this path.
Update Message
Marker Length Type
Unfeasible
Routes Length
Withdrawn
Routes
Attribute
Length
Path
Attributes
NLRI
Octets
Update Message
Marker Length Type
Unfeasible
Routes Length
Withdrawn
Routes
Attribute
Length
Path
Attributes
NLRI
Octets
Update Message Unreachable Routes
Information
NLRI
Information

 The NLRI is a list of <length, prefix> tuples.
◦ One tuple for each reachable destination.
◦ The prefix represents the reachable destination
◦ The prefix length represents the # of bits set in the subnet mask.
NLRI format
IP Address Subnet Mask NLRI
10.1.1.0 255.255.255.0 24, 10.1.1.0
192.24.160.0 255.255.224.0 19, 192.24.160.0

 A BGP notification message is
sent when an error condition
is detected.
◦ The BGP connection is
closed immediately after
this is sent.
 Notification messages include
an error code, an error
subcode, and data related to
the error.
16 2 1 1 1 Variable
Error
Sub-code
Diagnostic
Data
Octets

 Keepalive messages are sent between peers every 60 seconds (by default) to maintain connections.
 The message consists of only a message header (19 bytes).
◦ Hold time is three times the KEEPALIVE timer of 60 seconds.
◦ If the periodic timer = 0, no keepalives are sent.
◦ Recommended keepalive interval is one-third of the hold time interval.
Keepalive Message Type
16 2 1
Marker Length Type
Keepalive Message
Octets

 Path attributes are a set of BGP metrics describing the path to a network (route).
◦ BGP uses the path attributes to determine the best path to the networks.
◦ Some attributes are mandatory and automatically included in update messages while others are
manually configurable.
 BGP attributes can be used to enforce a routing policy.
 Configuring BGP attributes provides administrators with many more path control options.
◦ E.g., filter routing information, prefer certain paths, customize BGP’s behavior.
 A BGP update message includes a variable-length sequence of path attributes describing the route.
 A path attribute consists of three fields:
◦ Attribute type
◦ Attribute length
◦ Attribute value
Path Attributes
Marker Length Type
Unfeasible
Routes Length
Withdrawn
Routes
Attribute
Length
Path
Attributes
NLRI
Octets
Update Message
Path Attributes
Information
BGP Attribute Type
• Type code 1 ORIGIN
• Type code 2 AS_PATH
• Type code 3 NEXT_HOP
• Type code 4 MULTI_EXIT_DISC
• Type code 5 LOCAL_PREF
• Type code 6 ATOMIC_AGGREGATE
• Type code 7 AGGREGATOR
• Type code 8 Community (Cisco-defined)
• Type code 9 Originator-ID (Cisco-defined)
• Type code 10 Cluster list (Cisco-defined)

Path Attributes Within Update Message
Wireshark capture of an
update message
indicating the path
attributes to reach
network 172.19.0.0/16.

Attributes
 Some attributes are mandatory and automatically included in update messages while others are
manually configurable.
Attribute EBGP IBGP
AS_PATH Well-known Mandatory
Well-known
Mandatory
NEXT_HOP Well-known Mandatory
Well-known
Mandatory
ORIGIN Well-known Mandatory
Well-known
Mandatory
LOCAL_PREF Not allowed Well-known Discretionary
ATOMIC_AGGREGATE Well-known Discretionary Well-known Discretionary
AGGREGATOR Optional Transitive
Optional
Transitive
COMMUNITY Optional Transitive
Optional
Transitive
MULTI_EXIT_DISC Optional Nontransitive
Optional
Nontransitive
Automatically
included in
update
message
Can be
configured to
help provide
path control.

Path Attributes
 There are four different attribute types.
• Not all vendors recognize the same BGP attributes.

Well-Known Mandatory
 Attribute is recognized by all implementations of BGP and must appear in a BGP update message.
• If missing, a notification error will be generated.
 Well-known mandatory attributes ensures that all BGP implementations agree on a standard set of
attributes.

Well-Known Mandatory: AS_PATH
 The AS_PATH attribute contains a list of
AS numbers to reach a route.
 Whenever a route update passes through
an AS, the AS number is added to the
beginning of the AS_PATH attribute
before it is advertised to the next EBGP
neighbor.

 BGP always includes the AS_PATH
attribute in its update.
Well-Known Mandatory: AS_PATH

 The NEXT_HOP attribute indicates the IP address that is to be used to reach a destination.
 The IP address is the entry point of the next AS along the path to that destination network.
◦ Therefore, for EBGP, the next-hop address is the IP address of the neighbor that sent the update.
Well-Known Mandatory: NEXT_HOP

 The ORIGIN attribute defines the origin of the path which could be:
◦ IGP:
 The route is interior to the originating AS and normally occurs when a network command
is used to advertise the route via BGP.
 An origin of IGP is indicated with an “i” in the BGP table.
◦ EGP:
 (Obsolete) The route is learned via EGP which is considered a historic routing protocol and is
not supported on the Internet.
 An origin of EGP is indicated with an “e” in the BGP table.
◦ Incomplete:
 The route’s origin is unknown or is learned via some other means and usually occurs when a
route is redistributed into BGP.
 An incomplete origin is indicated with a “?” in the BGP table.
Well-Known Mandatory: ORIGIN

Well-Known Mandatory: ORIGIN
R1# show ip bgp
BGP table version is 24, local router ID is 172.16.1.2
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 192.208.10.0 192.208.10.5 0 0 300 i
*> 172.16.1.0 0.0.0.0 0 32768 i
<output omitted>
R1# show ip bgp
<output omitted>
*> 10.1.1.0/24 0.0.0.0 0 32768 ?
*> 192.168.1.0/24 10.1.1.2 84 32768 ?
*> 192.168.2.0/24 10.1.1.2 74 32768 ?
<output omitted>
i = Route generated
by the network
command.
? = Route generated
by unknown
method (usually
redistributed).

Well-Known Discretionary
 Attribute is recognized by all implementations of BGP but may not be sent in the BGP update
message.

 The Local Preference attribute provides an indication to the “local” routers in the AS about which
path is preferred to exit the AS.
◦ A path with a higher local preference is preferred.
◦ The default value for local preference on a Cisco router is 100.
 It is configured on a router and exchanged between IBGP routers.
◦ It is not passed to EBGP peers.
Well-Known Discretionary: LOCAL_PREF

Well-Known Discretionary: LOCAL_PREF
 Routers A and B are IBGP neighbors in AS 64520 and both receive updates about network
172.16.0.0 from different directions.
• The local preference on router A is set to 200.
• The local preference on router B is set to 150.
 Because the local preference for router A is higher, it is selected as the preferred exit point from
AS 64520.

 The bgp default local-preference command changes the default local preference
value.
◦ With this command, all IBGP routes that are advertised have the local preference set to the
value specified.
◦ If an EBGP neighbor receives a local preference value, the EBGP neighbor ignores it.
Configuring the Default Local Preference

 The Atomic Aggregate attribute is used to indicate that routes have been summarized.
◦ Attribute warns that the received information may not necessarily be the most complete route
information available.
 Attribute is set to either True or False with “true” alerting other BGP routers that multiple
destinations have been grouped into a single update.
◦ Router update includes its router ID and AS number along with the supernet route enabling
administrators to determine which BGP router is responsible for a particular instance of
aggregation.
◦ Tracing a supernet to its original "aggregator" may be necessary for troubleshooting purposes.
Well-Known Discretionary: ATOMIC_AGGREGATE

Optional Transitive
 Attribute may or may not be recognized by all BGP implementations.
 Because the attribute is transitive, BGP accepts and advertises the attribute even if it is not
recognized.

 The BGP community attribute can be used to filter incoming or outgoing routes.
◦ BGP routers can tag routes with an indicator (the community) and allow other routers to make
decisions based on that tag.
 If a router does not understand the concept of communities, it defers to the next router.
◦ However, if the router does understand the concept, it must be configured to propagate the
community; otherwise, communities are dropped by default.
 Communities are not restricted to one network or one AS, and they have no physical boundaries.
Optional Transitive: Community

Optional Nontransitive
 Attribute that may or may not be recognized by all BGP implementations.
 Whether or not the receiving BGP router recognizes the attribute, it is nontransitive and is not
passed along to other BGP peers.

 The Multiple Exit Discriminator (MED) attribute, also called the metric, provides a hint to external
neighbors about the preferred path into an AS that has multiple entry points.
◦ Lower MED is preferred over a higher MED!
 The MED is sent to EBGP peers and those routers propagate the MED within their AS.
◦ The routers within the AS use the MED, but do not pass it on to the next AS.
◦ When the same update is passed on to another AS, the metric will be set back to the default of 0.
 By using the MED attribute, BGP is the only protocol that can affect how routes are sent into an AS.
Optional Nontransitive: MED

Optional Nontransitive: MED
 Routers B and C include a MED attribute in the updates to router A.
• Router B MED attribute is set to 150.
• Router C MED attribute is set to 200.
 When A receives updates from B and C, it picks router B as the best next hop because of the lower
MED.

 The Weight attribute is a Cisco proprietary attribute.
 Similar in function to the local preference, the weight attribute applies when 1 router has multiple
exit points.
◦ Local preference is used when 2+ routers provide multiple exit points.
 It is configured locally on a router and is not propagated to any other routers.
◦ Routes with a higher weight are preferred when multiple routes exist to the same destination.
 The weight can have a value from 0 to 65535.
◦ Paths that the router originates have a weight of 32768 by default, and other paths have a
weight of 0 by default.
Cisco Weight Attribute

Cisco Weight Attribute
 Routers B and C learn about network 172.20.0.0 from AS 65250 and propagate the update to
router A.
• Therefore Router A has two ways to reach 172.20.0.0.
 Router A sets the weight of updates as follows:
• Updates coming from router B are set to 200
• Updates coming from router C are set to 150.
 Router A uses router B because of the higher weight.

 The BGP best path decision is based on the value of attributes that the update contains and other
BGP-configurable factors.
 BGP considers only synchronized routes with no AS loops and a valid next-hop address.
BGP Route Selection Process

• Prefer the route with highest
weight.
• Prefer the route with highest
LOCAL_PREF.
• Prefer the locally generated route
(network or aggregate routes).
• Prefer the route with the shortest
AS-PATH.
• Prefer the route with the lowest
ORIGIN (IGP<EGP<incomplete)
MED.
• Prefer the EBGP route over IBGP
route.
• Prefer the route through the closest
IGP neighbor.
• Prefer the oldest EBGP route.
neighbor BGP router ID value.
neighbor IP address.

 Prior to deploying a BGP routing solution, the following should be considered:
◦ IP addressing plan
◦ Network topology
◦ BGP relationship with service provider(s)
Planning to Deploy BGP

 The information necessary to implement BGP routing includes the following:
◦ The AS numbers of enterprise and service provider.
◦ The IP addresses of all the neighbors (peers) involved.
◦ The networks that are to be advertised into BGP
 In the implementation plan, basic BGP tasks include the following:
◦ Define the BGP process
◦ Establish the neighbor relationships
◦ Advertise the networks into BGP
Implementing Basic BGP

 After implementing BGP, verification should confirm proper deployment on each router.
◦ That the appropriate BGP neighbor relationships and adjacencies are established.
◦ That the BGP table is populated with the necessary information.
◦ That IP routing table is populated with the necessary information.
◦ That there is connectivity in the network between routers and to other devices.
◦ That BGP behaves as expected in a case of a topology change, by testing link failure and router
failure events.
Verifying BGP

 After a successful BGP deployment, the solution and verification process and results should be
◦ A topology map
◦ The IP addressing plan
◦ The autonomous system hierarchy
◦ The networks and interfaces included in BGP on each router
◦ The default and any special metrics configured
◦ The verification results.
Documenting

Enable BGP Routing
 Define BGP as the IP routing protocol.
Router(config)#
router bgp autonomous-system
 The autonomous-system value is either an internally generated number (if not
connecting to a provider network) or obtained from an ISP or RIR.
◦ It is a required parameter.
◦ It can be any positive integer in the range from 1 to 65535.
 Only one instance of BGP can be configured on the router at a single time.

 On Cisco routers there are two formats used to configure a 4-byte AS number:
◦ asplain: The Cisco implementation.
◦ asdot: The RFC 5396 implementation.
 Use the bgp asnotation dot command to configure.
 AS numbers must be written using the asdot format, or the regular expression match will fail.
 Note: The 4-byte AS number will not be used in this chapter; therefore, all examples use the 2-byte
AS numbering format.
4-byte AS numbers

Defining BGP Neighbors
 Identify peer router with which to establish a BGP session.
neighbor {ip-address | peer-group-name} remote-as
autonomous-system
 The ip-address is the destination address of the BGP peer.
◦ The address must be reachable before attempting to establish the BGP relationship.
 The autonomous-system value is used to identify if the session is with internal BGP
(IBGP) peers or with external BGP (EBGP) peers.
◦ If the value is the same as the router’s AS, then an IBGP session is attempted.
◦ If the value is not the same as the router’s AS, then an EBGP session is attempted.

 In BGP, neighbors are often configured with the same update policies.
 To simplify configuration and make updating more efficient, neighbors with the same update
policies can be grouped into peer groups.
◦ Recommended approach when there are many BGP peers.
 Instead of separately defining the same policies for each neighbor, a peer group can be defined
with these policies assigned to the peer group.
◦ Individual neighbors are then made members of the peer group.
◦ Members of the peer group inherit all the peer group’s configuration options.
◦ Only options that affect the inbound updates can be overridden.
BGP Peer Groups

Defining a BGP Peer Group
 Create a peer group on the local router.
neighbor peer-group-name peer-group
 The peer-group-name is the name of the BGP peer group to be created.
 The name is local to the router on which it is configured and is not passed to any other
router.

Assign Neighbors to the Peer Group
 Assign neighbors as part of the peer group.
neighbor ip-address peer-group peer-group-name
 The ip-address is the IP address of the neighbor that is to be assigned as a member of
the peer group.
 The peer-group-name must already exist.
◦ Note: The clear ip bgp peer-group peer-group-name EXEC command can
be used to reset the BGP connections for all members of a peer group.

Shut Down a BGP Neighbor
 To disable an existing BGP neighbor or peer group relationship.
neighbor {ip-address | peer-group-name} shutdown
 Useful when making major policy changes to a neighboring router.
 The command not only terminates the session, but also removes all associated routing
information.
 To re-enable the neighbor prepend the no keyword to the command.

 BGP does not accept unsolicited updates.
◦ It must be aware of every neighboring router and have a neighbor statement for it.
 For example, when a router creates and forwards a packet, the IP address of the outbound interface
is used as that packet’s source address by default.
◦ For BGP packets, this source IP address must match the address in the corresponding
neighbor statement on the other router or the routers will not establish the BGP session.
◦ This is not a problem for EBGP neighbors as they are typically directly connected.
IBGP Source IP Address Problem

IBGP Source IP Address Problem
 When multiple paths exist between IBGP neighbors, the BGP source address can cause problems:
• Router D uses the neighbor 10.3.3.1 remote-as 65102 command to establish a
relationship with A.
• However, router A is sending BGP packets to D via B therefore the source IP address of the
packets is 10.1.1.1.
• The IBGP session between A and D cannot be established because D does not recognize 10.1.1.1
as a BGP neighbor.

IBGP Source IP Address Solution
 Establish the IBGP session using a loopback interface.
neighbor {ip-address | peer-group-name} update-source
loopback interface-number
 Informs the router to use a loopback interface address for all BGP packets.
 Overrides the default source IP address for BGP packets.
 Typically only used with IBGP sessions.
 As an added bonus, physical interfaces can go down for any number of reasons but
loopbacks never fail.

IBGP Source IP Address Example
AS 65102
AS 65100
R1 R4
Lo0 192.168.2.2
.1
192.168.1.1172.16.1.1
R2 R3
10.1.1.0/24
10.2.2.0/24
.1 .2
.2
Lo0 192.168.3.3
AS 65101
EIGRP
R2(config)# router bgp 65101
R2(config-router)# neighbor 172.16.1.1 remote-as 65100
R2(config-router)# neighbor 192.168.3.3 update-source loopback0
R2(config-router)#
R3(config-router)#

EBGP Dual-Homed Problem
 R1 in AS 65102 is dual-homed with R2 in AS 65101.
 A problem can occur if R1 only uses a single neighbor statement pointing to
192.168.1.18 on R2 .
• If that link fails, the BGP session between these AS is lost, and no packets pass from one
autonomous system to the next, even though another link exists.
 A solution is configuring two neighbor statements on R1 pointing to 192.168.1.18 and
192.168.1.34.
• However, this doubles the BGP updates from R1 to R2.

EBGP Dual-Homed Solution
 The ideal solution is to:
• Use loopback addresses.
• Configure static routes to reach the loopback address of the other router.
• Configure the neighbor ebgp-multihop command to inform the BGP process that this
neighbor is more than one hop away.

Enable Multihop EBGP
 Increase the time-to-live (TTL) for EBGP connections.
neighbor {ip-address | peer-group-name} ebgp-multihop
[ttl]
 This command is of value when redundant paths exist between EBGP neighbors.
 The default ttl is 1, therefore BGP peers must be directly connected.
◦ The range is from 1 to 255 hops.
 Increasing the ttl enables BGP to establish EBGP connections beyond one hop and also
enables BGP to perform load balancing.

Multihop EBGP Example
AS 65102
Lo0 172.17.1.1
R1 R2
EBGP
EBGP
192.168.1.18 /28
Lo0 172.16.1.1
AS 65101
192.168.1. 34 /28
192.168.1.17 /28
192.168.1. 33 /28
R1(config-router)# neighbor 172.16.1.1 ebgp-multihop 2
R1(config)# ip route 172.16.1.1 255.255.255.255 192.168.1.18
R1(config)# ip route 172.16.1.1 255.255.255.255 192.168.1.34
R1(config)#
R2(config-router)# neighbor 172.17.1.1 ebgp-multihop 2
R2(config)# ip route 172.17.1.1 255.255.255.255 192.168.1.17
R2(config)# ip route 172.17.1.1 255.255.255.255 192.168.1.33
R2(config)#

 When an EBGP router receives an update from an EBGP neighbor and forwards the update to its
IBGP peers, the source IP address will still be that of the EBGP router.
◦ IBGP neighbors will have to be configured to reach that external IP address.
 Another solution is to override a router’s default behavior and force it to advertise itself as the
next-hop address for routes sent to a neighbor.
◦ To do so, use the neighbor next-hop-self router configuration command
Advertising EBGP Routes to IBGP Peers

neighbor next-hop-self Command
 Configure the router as the next hop for a BGP-speaking peer.
neighbor {ip-address | peer-group-name} next-hop-self
 The command forces BGP to advertise itself as the source of the routes.
 The ip-address identifies the peer router to which advertisements will be sent, with this
router identified as the next hop.
 This command is useful in unmeshed networks (such as Frame Relay) where BGP
neighbors may not have direct access to all other neighbors on the same IP subnet.

Next Hop Self Example
R2(config-router)# neighbor 192.168.3.3 next-hop-self
R2(config-router)#
AS 65102
AS 65100
R1 R4
Lo0 192.168.2.2
.1
192.168.1.1172.16.1.1
R2 R3
10.1.1.0/24
10.2.2.0/24
.1 .2
.2
Lo0 192.168.3.3
AS 65101
EIGRP

 Recall that the BGP synchronization rule states that:
◦ “A BGP router should not use, or advertise a route learned by IBGP, unless that route is local or is
learned from the IGP.”
 By default synchronization is disabled, therefore BGP can use and advertise to an external BGP
neighbor routes learned from an IBGP neighbor that are not present in the local routing table.
◦ Use the synchronization router configuration command to enable BGP synchronization
so that a router will not advertise routes in BGP until it learns them in an IGP.
◦ The no synchronization router configuration command disables synchronization.
BGP Synchronization

 Two options are available to advertise networks into BGP:
◦ The network command.
◦ Redistributing IGP routes into BGP.
 Note: Redistributing is not recommended because it could result in unstable BGP tables.
Defining Networks That BGP Advertises

Identify BGP Networks
 Enable BGP to advertise a network if it is present.
network network-number [mask network-mask] [route-map
map-tag]
 The BGP network command determines which networks this router advertises.
◦ Unlike IGPs, the command does not start BGP on specific interfaces.
 The mask parameter indicates that BGP-4 supports subnetting and supernetting.
◦ If the mask is not specified, this command announces only the classful network
 It is also important to note that the prefix must exactly match (address and mask) an entry
in the IP routing table.

 It is important to understand that any network (both address and mask) must exist in the routing
table for the network to be advertised in BGP.
 For example, to summarize many networks and advertise a CIDR block 192.168.0.0/16, configure:
network 192.168.0.0 mask 255.255.0.0
ip route 192.168.0.0 255.255.0.0 null0
 Now BGP can find an exact match in the routing table and announce the 192.168.0.0/16 network to
its neighbors.
◦ The advertised static route would never actually be used since BGP would contain longer prefix
matching routes in its routing table.
BGP Route Must Be in IP Routing Table

 BGP supports message digest 5 (MD5) neighbor authentication.
◦ MD5 sends a “message digest” (also called a “hash”), which is created using the key and a
message.
◦ The message digest is then sent instead of the key.
◦ The key itself is not sent, preventing it from being read by someone eavesdropping on the line
while it is being transmitted.
 To enable MD5 authentication on a TCP connection between two BGP peers, use the router
configuration command:
neighbor {ip-address | peer-group-name} password string
BGP Authentication

Enable MD5 authentication
 Enable MD5 authentication between two BGP peers.
neighbor {ip-address | peer-group-name} password
string
 This is the only command required to enable MD5 authentication.
 The string value is:
◦ Case-sensitive password of up to 25 characters.
◦ The first character cannot be a number.
◦ The string can contain any alphanumeric characters, including spaces.
◦ You cannot specify a password in the format number-space-anything.
◦ The space after the number can cause authentication to fail.

R1(config-router)# neighbor 10.64.0.2 password BGP-Pa55w0rd
R1(config-router)#
R2(config-router)# neighbor 10.64.0.1 password BGP-Pa55w0rd
R2(config-router)#
AS 65500
R1 R2
10.64.0.0 /24.1 .2
AS 65000

 If a router has a password configured for a neighbor, but the neighbor router does not have a
password configured, the following message will appear on the console screen:
%TCP-6-BADAUTH: No MD5 digest from 10.1.0.2(179) to 10.1.0.1(20236)
 Similarly, if the two routers have different passwords configured, the following will appear:
%TCP-6-BADAUTH: Invalid MD5 digest from 10.1.0.1(12293) to
10.1.0.2(179)
MD5 Configuration Problems

 When policies such as access lists or attributes are changed, the Cisco IOS applies changes on
only those updates received or sent after and not existing routes in the BGP and routing tables.
◦ It can take a long time for the policy to be applied to all networks.
 There are three ways to ensure that the policy change is immediately applied to all affected
prefixes and paths.
◦ Hard reset
◦ Soft reset (outbound and inbound)
◦ Route refresh
Clearing the BGP Session

Hard Reset of BGP Sessions
 Reset all BGP connections with this router.
Router#
clear ip bgp {* | neighbor-address}
 Entire BGP forwarding table is discarded.
 BGP session makes the transition from established to idle; everything must be relearned.
 When the neighbor-address value is used, it resets only a single neighbor and BGP
session. Everything from this neighbor must be relearned.
◦ It is less severe than clear ip bgp *.

Soft Reset Outbound
 Resets all BGP connections without loss of routes.
Router#
clear ip bgp {* | neighbor-address} [soft out]
 The connection remains established and the command does not reset the BGP session.
◦ Instead the router creates a new update and sends the whole table to the specified
neighbors.
 This update includes withdrawal commands for networks that the neighbor will not see
anymore based on the new outbound policy.
 This option is highly recommended when you are changing outbound policy.

Soft Reset Inbound: Method #1
 Two commands are required.
neighbor {ip-address} soft-reconfiguration inbound
 Use this command when changes need to be made without forcing the other side to
resend everything.
 It causes the BGP router to retain an unfiltered table of what a neighbor had sent but can
be memory intensive.
Router#
clear ip bgp {* | neighbor-address} [soft in]
 Causes the router to use the stored unfiltered table to generate new inbound updates and
the new results are placed in the BGP forwarding database.

Soft Reset Inbound: Method #2
 Also called route refresh.
Router#
clear ip bgp {* | neighbor-address} [soft in | in]
 This dynamically soft resets inbound updates.
 Unlike method #1, this method requires no preconfiguration and requires significantly less
memory.

Command Description
show ip bgp neighbors
{address} received-routes
Displays all received routes (both accepted and
rejected) from the specified neighbor.
{address} routes
Displays all routes that are received and accepted
from the specified neighbor.
This output is a subset of the output displayed by the
received-routes keyword.
show ip bgp Displays entries in the BGP table.
{address} advertised-routes
Displays all BGP routes that have been advertised to
neighbors.
Monitoring Received BGP Routes

BGP Configuration Example #1
R1(config-router)#
R2(config-router)#
AS 65000
R1 R2
10.1.1.0.1 .2
AS 64520
172.17.0.0172.16.0.0

BGP Configuration Example #2
R2(config-router)# network 10.2.2.0 mask 255.255.255.0
R2(config-router)#
AS 65020
R2
10.1.1.0 /24
.2
AS 65010
10.2.2.0 /24
R1
.1
R3
10.3.3.0 /24.2
Lo0 10.4.4.4
.1
Lo0 10.5.5.5

BGP Without Peer Group Example
R1(config-router)# neighbor 192.168.24.1 update-source loopback 0
R1(config-router)# neighbor 192.168.24.1 distribute-list 20 out
R1(config-router)#
R1(config-router)#
R1(config-router)#

BGP With Peer Group Example
R1(config-router)# neighbor INTERNAL peer-group
R1(config-router)# neighbor INTERNAL remote-as 65100
R1(config-router)# neighbor INTERNAL update-source loopback 0
R1(config-router)# neighbor INTERNAL next-hop-self
R1(config-router)# neighbor INTERNAL distribute-list 20 out
R1(config-router)# neighbor 192.168.24.1 peer-group INTERNAL
R1(config-router)#

IBGP and EBGP Example
R2(config-router)# no synchronization
R2(config-router)#
172.16.30.0172.16.20.0
Lo0 192.168.2.2 /32
.2
R2 R3
192.168.1.0 /24
192.168.4.0 /24
.2 .3
.3
Lo0 192.168.3.3 /32
AS 65000
10.1.1.0 /24.1
R1
AS 64520
.2
172.16.10.0

Verifying and
Troubleshooting
BGP

Command Description
show ip bgp
Displays entries in the BGP table.
Specify a network number to get more specific information
about a particular network.
show ip bgp neighbors Displays detailed information about the TCP and BGP
connections to neighbors.
show ip bgp summary Displays the status of all BGP connections.
{address} advertised-
routes
Displays all BGP routes that have been advertised to
neighbors.
show ip bgp rib-failure
Displays BGP routes that were not installed in the routing
information base (RIB), and the reason that they were not
installed.
debug ip bgp [dampening
| events | keepalives |
updates]
Verifying and Troubleshooting BGP

Verifying BGP: show ip bgp
R1# show ip bgp
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal, r RIB-failure, S Stale
*> 10.1.0.0/24 0.0.0.0 0 32768 i
* i 10.1.0.2 0 100 0 i
*> 10.1.1.0/24 0.0.0.0 0 32768 i
*>i10.1.2.0/24 10.1.0.2 0 100 0 i
*> 10.97.97.0/24 172.31.1.3 0 64998 64997 i
* 172.31.11.4 0 64999 64997 i
* i 172.31.11.4 0 100 0 64999 64997 i
*> 10.254.0.0/24 172.31.1.3 0 0 64998 i
* 172.31.11.4 0 64999 64998 i
* i 172.31.1.3 0 100 0 64998 i
r> 172.31.1.0/24 172.31.1.3 0 0 64998 i
r 172.31.11.4 0 64999 64998 i
r i 172.31.1.3 0 100 0 64998 i
*> 172.31.2.0/24 172.31.1.3 0 0 64998 i
Display the BGP topology database (the BGP table).
The status codes are shown in
the first column of each line of
output.
- * means that the next-hop
address (in the fifth column) is
valid.
- r means a RIB failure and the
route was not installed in the
RIB.
The last column displays the ORIGIN attribute).
- i means the original router probably used a
network command to introduce this network
into BGP.
- ? means the route was probably redistributed
from an IGP into the BGP process.
The third column is either blank
or has an “i” in it.
- If it has an i, an IBGP
neighbor advertised this route
to this router.
- If it is blank, BGP learned that
route from an external peer.
A > in the second column
indicates the best path for a
route selected by BGP.
This route is offered to the IP
routing table.
The Path section lists the AS
path. The last AS # is the
originating AS.
If blank the route is from the
current autonomous system.
This section lists three BGP
path attributes: metric (MED),
local preference, and weight.

Verifying BGP: show ip bgp rib-failure
 Displays BGP routes that were not installed in the RIB and the reason that they were not
installed.
 In this example, the displayed routes were not installed because a route or routes with a better
administrative distance already existed in the RIB.
R1# show ip bgp rib-failure
Network Next Hop RIB-failure RIB-NH Matches
172.31.1.0/24 172.31.1.3 Higher admin distance n/a
172.31.11.0/24 172.31.11.4 Higher admin distance n/a

Verifying BGP: show ip bgp summary
R1# show ip bgp summary
BGP router identifier 10.1.1.1, local AS number 65001
BGP table version is 124, main routing table version 124
9 network entries using 1053 bytes of memory
22 path entries using 1144 bytes of memory
12/5 BGP path/bestpath attribute entries using 1488 bytes of memory
6 BGP AS-PATH entries using 144 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 3829 total bytes of memory
BGP activity 58/49 prefixes, 72/50 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.1.0.2 4 65001 11 11 124 0 0 00:02:28 8
172.31.1.3 4 64998 21 18 124 0 0 00:01:13 6
172.31.11.4 4 64999 11 10 124 0 0 00:01:11 6
Verify the BGP neighbor relationship.

Verifying BGP: debug ip bgp updates
R1# debug ip bgp updates
Mobile router debugging is on for address family: IPv4 Unicast
R1# clear ip bgp 10.1.0.2
<output omitted>
*May 24 11:06:41.309: %BGP-5-ADJCHANGE: neighbor 10.1.0.2 Up
*May 24 11:06:41.309: BGP(0): 10.1.0.2 send UPDATE (format) 10.1.1.0/24, next 10.1.0.1, metric 0,
path Local
*May 24 11:06:41.309: BGP(0): 10.1.0.2 send UPDATE (prepend, chgflags: 0x0) 10.1.0.0/24, next
10.1.0.1, metric 0, path Local
*May 24 11:06:41.309: BGP(0): 10.1.0.2 NEXT_HOP part 1 net 10.97.97.0/24, next 172.31.11.4
*May 24 11:06:41.309: BGP(0): 10.1.0.2 send UPDATE (format) 10.97.97.0/24, next 172.31.11.4, metric
0, path 64999 64997
*May 24 11:06:41.309: BGP(0): 10.1.0.2 NEXT_HOP part 1 net 172.31.22.0/24, next 172.31.11.4
*May 24 11:06:41.309: BGP(0): 10.1.0.2 send UPDATE (format) 172.31.22.0/24, next 172.31.11.4,
metric 0, path 64999
<output omitted>
*May 24 11:06:41.349: BGP(0): 10.1.0.2 rcvd UPDATE w/ attr: nexthop 10.1.0.2, origin i, localpref
100, metric 0
*May 24 11:06:41.349: BGP(0): 10.1.0.2 rcvd 10.1.2.0/24
*May 24 11:06:41.349: BGP(0): 10.1.0.2 rcvd 10.1.0.0/24

 BGP is a state machine that takes a router through the following states with its neighbors:
◦ Idle
◦ Connect
◦ Open sent
◦ Open confirm
◦ Established
 The Idle state begins once the neighbor command is configured.
BGP States

VerifyingBGP:show ip bgp neighbors
R1# show ip bgp neighbors
BGP neighbor is 172.31.1.3, remote AS 64998, external link
BGP version 4, remote router ID 172.31.2.3
BGP state = Established, up for 00:19:10
Last read 00:00:10, last write 00:00:10, hold time is 180, keepalive
interval is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(old & new)
Address family IPv4 Unicast: advertised and received
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 7 7
Notifications: 0 0
Updates: 13 38
<output omitted>

Basic BGP Path
Manipulation Using
Route Maps

 In Chapter 4, Policy Based Routing (PBR) was used for redistribution.
◦ Route maps are implemented using the redistribute command.
 In Chapter 5, route maps were used to define a routing policy other than basic destination-based
routing using the routing table.
◦ Route maps are implemented using the ip policy route-map command.
 In this chapter, route maps will be used with BGP to assign or alter BGP attributes.
◦ Route maps are implemented using the neighbor route-map command.
Route Maps and BGP

Sample implementation plan:
 Define and name the route map with the route-map command.
◦ Define the conditions to match (the match statements).
◦ Define the action to be taken when there is a match (the set statements).
 Define which attribute to alter using the neighbor route-map router configuration
command.
◦ Filters incoming or outgoing BGP routes.
 Verify results.
Configuring Route Maps in BGP

neighbor {ip-address | peer-group-name} route-map map-name
{in | out}
 Applies the route-map to filter incoming or outgoing BGP routes to a
neighbor.
Implementing Route Maps in BGP
route-map map-tag [permit | deny] [sequence-number]
Router(config)#
 Defines the route map conditions.
match {criteria}
Router(config-route-map)#
 Defines the criteria to match.
set {actions}
 Defines the action to be taken on a match.

match Commands Used in BGP
Command Description
match as-path Matches the AS_PATH attribute
match ip address
Matches any routes that have a destination network number address
that is permitted by a standard or extended ACL
match metric Matches routes with the metric specified
match community Matches a BGP community
match interface
Matches any routes that have the next hop out of one of the interfaces
specified
match ip next-hop
Matches any routes that have a next-hop router address that is passed
by one of the ACLs specified
match ip route-source
Matches routes that have been advertised by routers and access
servers at the address that is specified by the ACLs
match route-type Matches routes of the specified type
match tag Matches tag of a route
* Partial list

match as-path Command
 Match a BGP autonomous system path access list.
match as-path path-list-number
 The path-list-number is the AS path access list.
◦ It can be an integer from 1 to 199.
 The value set by this command overrides global values.

match ip-address Command
 Specify criteria to be matched using ACLs or prefix lists.
match ip address {access-list-number | name} [...access-
list-number | name] | prefix-list prefix-list-name
[..prefix-list-name]
access-list-number | name
The number or name of a standard or
extended access list to be used to test
incoming packets.
If multiple access lists are specified,
matching any one results in a match.
prefix-list prefix-list-name
Specifies the name of a prefix list to be used
to test packets.
If multiple prefix lists are specified, matching
any one results in a match.

set Commands Used in BGP
Command Description
set weight Sets the BGP weight value
set local-preference Sets the LOCAL-PREF attribute value
set as-path Modifes an AS path for BGP routes
set origin Sets the ORIGIN attribute value
set metric Sets the Multi-Exit_Disc (MED) value
set community Sets the BGP communities attribute
set automatic-tag Computes automatically the tag value
set ip next-hop Indicates which IP address to output packets
set interface Indicates which interface to output packets
set ip default next-
hop
Indicates which default IP address to use to output packets
set default interface Indicates which default interface to use to output packets
* Partial list

set weight Command
 Specify the BGP weight for the routing table.
set weight number
 The number is the weight value.
◦ It can be an integer ranging from 0 to 65535.
 The implemented weight is based on the first matched AS path.
 Weights assigned with this command override the weights assigned using the neighbor
weight command.

set local-preference Command
 Specify a preference value for the AS path.
set local-preference number-value
 The number-value is the preference value.
 An integer from 0 to 4294967295.
 Default 100.

set as-path Command
 Modify an AS path for BGP routes.
set as-path {tag | prepend as-path-string}
tag
Converts the tag of a route into an autonomous system path.
Applies only when redistributing routes into BGP.
prepend
Appends the string following the keyword prepend to the AS
path of the route that is matched by the route map.
Applies to inbound and outbound BGP route maps.
as-path-string
AS number to prepend to the AS_PATH attribute.
The range of values for this argument is 1 to 65535.
Up to 10 AS numbers can be entered.

set metric Command
 Specify a preference value for the AS path.
set metric metric-value
 The metric-value is use to set the MED attribute.
 An integer from 0 to 294967295.

 Unlike IGPs, BGP was never designed to choose the quickest path.
 BGP was designed to manipulate traffic flow to maximize or minimize bandwidth use.
BGP Path Manipulation

BGP Without Routing Policy Example #1
 In this example consider that:
• R1 is using 60% of its outbound bandwidth to AS 65004.
 Traffic should be diverted using the local preference attribute.
• The weight attribute could not be used in this scenario since there are two edge routers.`

Which traffic should be re-routed?
 To determine which path to manipulate, perform a traffic analysis on Internet-bound traffic by
examining the most heavily visited addresses, web pages, or domain names.
• Examine network management records or accounting information.
 If a heavily accessed traffic pattern is identified, a route map could be used to divert that traffic
over the lesser used links

BGP With Routing Policy Example #1
 For example, assume that 35% of all traffic from AS 65001 has been going to
http://www.cisco.com.
• The administrator does a reverse DNS lookup and obtains the Cisco IP address and AS
number.
 A route map can be used to change the local preference to manipulate packets destined to
Cisco’s network over the less used links.

BGP Routing Policy Example #2
 Notice that the inbound load to R3 (75%) is much higher in bandwidth utilization than the
inbound load to R1 (10%).
 The BGP MED attribute can be used to manipulate how traffic enters autonomous system
65001.
 For example, R1 in AS 65001 can announce a lower MED for routes to network
192.168.25.0/24 to AS 65004 than R3 announces.

BGP Routing Policy Example #2
 Keep in mind that the MED is considered a
recommendation because the receiving autonomous
system can override it by manipulating another variable
that is considered before the MED is evaluated.
 For example, R2 and R4 in AS 65004 could be
configured with their own local preference policy which
would override the MED recommendation from AS
65001.
1. Prefer highest Weight
2. Prefer highest LOCAL_PREF
3. Prefer locally generated routes
4. Prefer shortest AS_PATH
5. Prefer lowest ORIGIN (IGP < EGP <
incomplete)
6. Prefer lowest MED
7. Prefer EBGP over IBGP
8. Prefer routes through closest IGP neighbor
9. Prefer routes with lowest BGP router ID
10.Prefer routes with lowest neighbor IP address

Change the Weight
 The weight attribute is used
only when one router is
multihomed and determines
the best path to leave the AS.
◦ Only the local router is
influenced.
◦ Higher weight routes are
preferred.
 There are two ways to alter
the route weight:
◦ To change the weight for all
updates from a neighbor use
the neighbor weight router
◦ To change the weight of
specific routes / as path, use
route maps.
5. Prefer lowest ORIGIN (IGP < EGP <
incomplete)
8. Prefer routes through closest IGP
neighbor
9. Prefer routes with lowest BGP
router ID
10. Prefer routes with lowest neighbor
IP address

Changing the Default Weight Example
 Assign a default weight to all routes from a peer.
neighbor {ip-address | peer-group-name} weight number
 Routes learned through another BGP peer have a default weight of 0 and routes sourced by
the local router have a default weight of 32768.
 The number is the weight to assign.
◦ Acceptable values are from 0 to 65535.
 The route with the highest weight will be chosen as the preferred route when multiple
routes are available to a particular network.
 Note: The weights assigned with the set weight route-map command override the
weights assigned using the neighbor weight command.

Changing Weight with Route Map Example
 In this example consider that:
• The routing policy dictates that for any network originated by AS 65020, use the path to AS
65030 as the primary way out of AS 65040.
• If R1 needs to access routes connected to R3, then it goes through R2.
 This can be achieved by placing a higher weight (150) on all incoming announcements from AS
65030 (10.0.0.1), which carry the information about the network originated in AS 65020.

Changing Weight with Route Map Example
R1(config)# route-map SET-WEIGHT permit 10
R1(config-route-map)# match as-path 10
R1(config-route-map)# set weight 150
R1(config-route-map)#
R1(config-route-map)# route-map SET-WEIGHT permit 20
R1(config-route-map)# set weight 100
R1(config-route-map)# exit
R1(config)# ip as-path access-list 10 permit _65020$
R1(config)#
R1(config-router)# neighbor 10.0.0.1 route-map SET-WEIGHT in

Configure an Autonomous System ACL
 Configure an autonomous system path filter.
ip as-path access-list acl-number {permit | deny}
regexp
 Similar to an IP ACL, this command is used to configure an AS path filter using a regular
expression .
 The acl-number is a value from 1 to 500 that specifies the AS_PATH access list number.
 The regexp regular expression defines the AS-path filter.

 Atom: A single character.
◦ . matches any single character.
◦ ^ matches the start of the input string.
◦ $ matches the end of the input string.
◦ matches the character.
 Piece: one of these symbols
◦ * matches 0 or more sequences of the atom.
◦ + matches 1 or more sequences of the atom.
◦ ? matches the atom or the null string.
 Branch: 1 or more concatenated pieces.
 Range: A sequence of characters within square brackets.
◦ Example is [abcd].
Regular Expression Syntax

Regular Expression Examples
Regular Expression Resulting Expression
a*
Expression indicates any occurrence of the letter "a", which
includes none
a+
indicates that at least one occurrence of the letter "a" must be
present
ab?a Expression matches "aa" or "aba".
_100_ Expression means via AS100.
_100$ Expression indicates an origin of AS100.
^100 .* Expression indicates transmission from AS100
^$ Expression indicates origination from this AS

Change the Local Preference
 The local preference is used only within an AS
(between IBGP speakers) to determine the
best path to leave the AS.
◦ Higher values are preferred.
◦ The local preference is set to 100 by
default.
 There are two ways to alter the local
preference:
◦ To change the default local-preference for
all routes advertised by the router use the
bgp default local-preference
value router configuration command.
◦ To change the local-preference of specific
routes / as path, use route maps.
5. Prefer lowest ORIGIN (IGP < EGP
< incomplete)
8. Prefer routes through closest IGP
neighbor
router ID
10. Prefer routes with lowest
neighbor IP address

Setting Default Local Preference Example
 Change the default local preference for outgoing routes.
bgp default local-preference number
 The local preference attribute applies a degree of preference to a route during the BGP best
path selection process.
◦ The attribute is exchanged only between iBGP peers.
◦ The route with the highest local preference is preferred.
 The number is the local preference value from 0 to 4294967295.
◦ Cisco IOS software applies a local preference value of 100.
 Note: The local preference assigned with the set local-preference route-map
command override the weights assigned using this command.

 The BGP routing policy in this example dictates that:
• The default local preference for all routes on R1 should be set to 200.
• The default local preference for all routes on R2 should be set to 500.

 The resulting configuration makes the IBGP routers in AS 65001 send all Internet bound traffic
to R2, but the R1 to ISP1 link is underutilized.
• Route maps could be configured to select specific routes to have a higher local preference.
R2(config-router)# bgp default local-preference 500
R2(config-router)#
R1(config-router)# bgp default local-preference 200
R1(config-router)#

Local Preference and Route Map Example
 The BGP routing policy results in the following:
• All routes have a weight of 0 and a default local preference of 100.
• BGP uses the shortest AS-path to select the best routes as follows:
• For network 172.16.0.0, the shortest AS-path is through ISP1.

R3# show ip bgp
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r
RIB-failure, S Stale
* i172.16.0.0 172.20.50.1 100 0 65005 65004 65003 i
*>i 192.168.28.1 100 0 65002 65003 i
*>i172.24.0.0 172.20.50.1 100 0 65005 i
* i 192.168.28.1 100 0 65002 65003 65004 65005 i
*>i172.30.0.0 172.20.50.1 100 0 65005 65004 i
* i 192.168.28.1 100 0 65002 65003 65004i

 A traffic analysis reveals the following traffic patterns:
• 10% of traffic flows from R1 to ISP1 to network 172.16.0.0.
• 50% of Internet traffic flow from R2 to ISP2 to networks network 172.24.0.0 and network
172.30.0.0.
• The remaining 40 percent is going to other destinations.
 A solution is to use route maps to divert traffic to 172.30.0.0 through R1.

 R1(config)# access-list 65 permit 172.30.0.0 0.0.255.255
 R1(config)#
 R1(config)# route-map LOCAL_PREF permit 10
 R1(config-route-map)# match ip address 65
 R1(config-route-map)# set local-preference 400
 R1(config-route-map)#
 R1(config-route-map)# route-map LOCAL_PREF permit 20
 R1(config-route-map)# exit

R1(config-router)# neighbor 192.168.28.1 route-map LOCAL_PREF in
R1(config)#

R3# show ip bgp
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r
RIB-failure, S Stale
* i172.16.0.0 172.20.50.1 100 0 65005 65004 65003 i
*>i 192.168.28.1 100 0 65002 65003 i
*>i172.24.0.0 172.20.50.1 100 0 65005 i
* i 192.168.28.1 100 0 65002 65003 65004 65005 i
* i172.30.0.0 172.20.50.1 100 0 65005 65004 i
*>i 192.168.28.1 400 0 65002 65003 65004i

Modifying the AS Path
 By default, if no BGP path selection tools
are configured to influence traffic flow (i.e.
weight, local-preference), BGP uses the
shortest AS path, regardless of available
bandwidth.
 To influence the path selection based on
the AS_PATH, configure AS-path
prepending.
◦ The AS path is extended with multiple
copies of the AS number of the sender
making it appear longer.
5. Prefer lowest ORIGIN (IGP <
EGP < incomplete)
8. Prefer routes through closest
IGP neighbor
router ID
neighbor IP address

Modifying the AS Path Example
• Traffic entering AS 65040 should be through R6 in AS 65030 and not R4 in AS 65010.
 One way to do this is make R1 advertise the AS 65040 networks with a less desirable AS path by
configuring AS-path prepending.

Modifying the AS Path Example
R1(config)# route-map SET-AS-PATH permit 10
R1(config-route-map)# set as-path prepend 65040 65040 65040
R1(config-router)# neighbor 172.16.1.1 route-map SET-AS-PATH out
R1(config)#

Setting the MED
 MED is used to decide how to enter an AS when
multiple paths exist.
◦ When comparing MED values for the same
destination network in the BGP path-selection
process, the lowest MED value is preferred.
◦ Default is 0.
 However, because MED is evaluated late in the
BGP path-selection process, it usually has no
influence.
 There are two ways to alter the MED:
◦ To change the MED for all routes use the
default-metric router configuration
command.
◦ To change the MED of specific routes / as
path, use route maps.
2. Prefer highest
LOCAL_PREF
3. Prefer locally generated
routes
5. Prefer lowest ORIGIN (IGP
< EGP < incomplete)
8. Prefer routes through
closest IGP neighbor
BGP router ID
neighbor IP address

Setting the Default MED Example
• The default MED of R1 should be changed to 1001.
• The default MED of R2 should be changed to 99.

Setting the Default MED Example
 The results are that the inbound bandwidth utilization on:
• R1 to ISP1 link has decreased to almost nothing except for BGP routing updates.
• R2 to ISP2 link has increased due to all returning packets from AS 65004.
• A better solution is to have route maps configured that will make some networks have a
lower MED through R1 and other networks to have a lower MED through R2.
R1(config-router)#
R2(config-router)#

Setting the MED with Route Maps Example
R1(config)# access-list 66 permit 192.168.25.0 0.0.0.255
R1(config)#
R1(config)# route-map MED-65004 permit 10
R1(config-route-map)# match ip address 66
R1(config-route-map)# set metric 100
R1(config-route-map)# route-map MED-65004 permit 100
R1(config)#

R1(config-router)# neighbor 192.168.28.1 route-map MED-65004 out
R1(config-router)#exit

R2(config)#
R2(config)# route-map MED-65004 permit 10
R2(config-route-map)# match ip address 66
R2(config-route-map)# route-map MED-65004 permit 100
R2(config)#

R2(config-router)# neighbor 172.20.50.1 route-map MED-65004 out
R2(config)#

 ISP3# show ip bgp
 BGP table version is 7, local router ID is 192.168.1.1
 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale
 Origin codes: i - IGP, e - EGP, ? - incomplete
 Network Next Hop Metric LocPrf Weight Path
 *>i192.168.24.0 172.20.50.2 100 100 0 65001 i
 * i 192.168.28.2 200 100 0 65001 i
 * i192.168.25.0 172.20.50.2 200 100 0 65001 i
 *>i 192.168.28.2 100 100 0 65001 i
 * i192.168.26.0 172.20.50.2 200 100 0 65001 i
 *>i 192.168.28.2 100 100 0 65001 i

 BGP terminology and concepts, including:
◦ BGP’s use between autonomous systems.
◦ The range of private AS numbers: 64512 to 65535.
◦ Requirements for Enterprise connection to an ISP including public IP address space, link type
and bandwidth, routing protocol, and connectivity redundancy.
 The four connection link type options: circuit emulation, MPLS VPNs, static routes, and BGP.
 The four connection redundancy types: Single-homed, Dual-homed, Multihomed, Dual-
multihomed.
 BGP neighbor (peer) relationships:
◦ IBGP is when BGP runs between routers in the same AS
◦ EBGP is when BGP runs between routers that are in different autonomous systems; EBGP
neighbors are typically directly connected
 Multihoming options:
◦ Each ISP passes only a default route to the AS.
◦ Each ISP passes only a default route and provider-owned specific routes to the AS.
◦ Each ISP passes all routes to the AS.
 BGP's loop free guarantee, because it does not accept a routing update that already includes its AS
number in the path list.
 When to use BGP and when not to use BGP.
Summary

 BGP’s classification as a path vector protocol and its use of TCP protocol 179.
 The use of full-mesh IBGP on all routers in the transit path within the AS.
 The BGP synchronization rule.
 The three tables used by BGP: the BGP table, IP routing table, and BGP neighbor table.
 The four BGP message types: open, keepalive, update, and notification..
 BGP attributes: well-known or optional, mandatory or discretionary, and transitive or
nontransitive.
 The BGP Well-known attributes including: AS-path, next-hop and origin.
 The BGP Well-known discretionary attributes including: local-preference, atomic aggregate.
 The BGP optional transitive attributes including: aggregator and community.
 The BGP optional nontransitive attributes including the MED.
 The Cisco specific weight attribute was also discussed.
 The 11-step BGP route selection decision process.
 BGP configuration commands.
 BGP verification commands.
 BGP path manipulation commands.
Summary

Routing Protocol in detail

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