Exploration routing chapter_11

OSPF

Routing Protocols and
Concepts – Chapter 11

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© 2007 Cisco Systems, Inc. All rights reserved.

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Objectives


Describe the background and basic features of OSPF



Identify and apply the basic OSPF configuration
commands



Describe, modify and calculate the metric used by
OSPF



Describe the Designated Router/Backup Designated
Router (DR/BDR) election process in multiaccess
networks



Describe the uses of additional configuration
commands in OSPF

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Introduction

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Introduction to OSPF
Background of OSPF
 Began in 1987
 1989 OSPFv1 released in RFC 1131
This version was experimental & never deployed
 1991 OSPFv2 released in RFC 1247
 1998 OSPFv2 updated in RFC 2328
 1999 OSPFv3 published in RFC 2740

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OSPF Message Encapsulation
 OSPF packet type
There exist 5 types
 OSPF packet header
Contains - Router ID and
area ID and Type code for
OSPF packet type
 IP packet header
Contains - Source IP
address, Destination IP
address, & Protocol field
set to 89
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OSPF Message Encapsulation
 Data link frame header
Contains - Source MAC address and Destination
MAC address

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OSPF Packet Types

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Hello Protocol
 OSPF Hello Packet
–Purpose of Hello Packet
 Discover OSPF neighbors & establish adjacencies
 Advertise guidelines on which routers must agree to become
neighbors
 Used by multi-access networks to elect a designated router and a
backup designated router

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 Hello Packets continued
Contents of a Hello Packet
router ID of transmitting router

 OSPF Hello Intervals
–Usually multicast (224.0.0.5)
–Sent every 30 seconds for NBMA segments

 OSPF Dead Intervals
–This is the time that must transpire
before the neighbor is considered
down
–Default time is 4 times
the hello interval

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 Hello protocol packets contain information that is used
in electing
-Designated Router (DR)
 DR is responsible for updating all other OSPF routers
-Backup Designated Router (BDR)
 This router takes over DR’s responsibilities if DR fails

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OSPF Link-state Updates

 Purpose of a Link State Update (LSU)
Used to deliver link state advertisements
 Purpose of a Link State Advertisement (LSA)
Contains information about neighbors & path costs

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OSPF Algorithm
 OSPF routers build &
maintain link-state
database containing LSA
received from other routers
–Information found in
database is utilized upon
execution of Dijkstra SPF
algorithm
–SPF algorithm used to
create SPF tree
–SPF tree used to populate
routing table

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Administrative Distance
 Default Administrative Distance for OSPF is 110

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 OSPF Authentication
–Purpose is to encrypt & 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 password or authentication information

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Basic OSPF Configuration
Lab Topology
 Topology used for this chapter
Discontiguous IP addressing scheme
Since OSPF is a classless routing protocol the subnet mask is
configured in

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The router ospf command
 To enable OSPF on a router use the following
command
R1(config)#router ospf process-id
Process id
 A locally significant number between 1 and 65535
-this means it does not have to match other OSPF
routers

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 OSPF network command
-Requires entering: network address
wildcard mask - the inverse of the subnet
mask
area-id - area-id refers to the OSPF area.
OSPF area is a group of routers that
share link state information
-Example: Router(config-router)#network network-address
wildcard-ask area area-id

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 Router ID
–This is an IP address used to identify a router
–3 criteria for deriving the router ID
Use IP address configured with OSPF router-id command
-Takes precedence over loopback and physical interface
addresses
If router-id command not used then router chooses highest
IP address of any loopback interfaces
If no loopback interfaces are configured then the highest IP
address on any active interface is used

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OSPF Router ID
 Commands used to verify current router ID
–Show ip protocols
–Show ip ospf
–Show ip ospf interface

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OSPF Router ID
 Router ID & Loopback addresses
-Highest loopback address will be used as router ID if router-id
command isn’t used
-Advantage of using loopback address
the loopback interface cannot fail  OSPF stability

 The OSPF router-id command
–Introduced in IOS 12.0
–Command syntax
Router(config)#router ospfprocess-id
Router(config-router)#router-idip-address

 Modifying the Router ID
–Use the command Router#clear ip ospf process
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Verifying OSPF
 Use the show ip ospf command to verify & trouble
shoot OSPF networks
Command will display the following:
 Neighbor adjacency
-No adjacency indicated by Neighboring router’s Router ID is not displayed
A state of full is not displayed
-Consequence of no adjacencyNo link state information exchanged
Inaccurate SPF trees & routing tables

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Verifying OSPF - Additional Commands
Command

Description

Show ip protocols

Displays OSPF process ID,
router ID, networks router is
advertising & administrative
distance

Show ip ospf

Displays OSPF process ID,
router ID, OSPF area information
& the last time SPF algorithm
calculated

Show ip ospf interface

Displays hello interval and dead
interval

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Examining the routing table
 Use the show ip route command to display the routing
table
-An “O’ at the beginning of a route indicates that the router
source is OSPF
-Note OSPF does not automatically summarize at major
network boundaries

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OSPF Metric
 OSPF uses cost as the metric for determining the
best route
-The best route will have the lowest cost
-Cost is based on bandwidth of an interface
Cost is calculated using the formula

108 / bandwidth
-Reference bandwidth
defaults to 100Mbps
can be modified using
auto-cost reference-bandwidth command

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OSPF Metric
 COST of an OSPF route
Is the accumulated value from one router to the next

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OSPF Metric
 Usually the actual speed of a link is different than the
default bandwidth
–This makes it imperative that the bandwidth value reflects
link’s actual speed
Reason: so routing table has best path information

 The show interface command will display interface’s
bandwidth
-Most serial link default to 1.544Mbps

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Modifying the Cost of a link
 Both sides of a serial link should be configured with the
same bandwidth
–Commands used to modify bandwidth value
Bandwidth command
–Example: Router(config-if)#bandwidthbandwidth-kbps
ip ospf cost command – allows you to directly specify
interface cost
-Example:R1(config)#interface serial 0/0/0
R1(config-if)#ip ospf cost 1562

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Modifying the Cost of the link
 Difference between bandwidth command & the ip ospf
cost command
–Ip ospf cost command
Sets cost to a specific value
–Bandwidth command
Link cost is calculated

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OSPF and Multiaccess Networks
Challenges in Multiaccess Networks
 OSPF defines five network types:
–Point-to-point
–Broadcast Multiaccess
–Nonbroadcast Multiaccess (NBMA)
–Point-to-multipoint
–Virtual links

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OSPF in Multiaccess Networks
 2 challenges presented by multiaccess networks
–Multiple adjacencies
–Extensive LSA flooding

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 Extensive flooding of LSAs
For every LSA sent out there must be an acknowledgement of
receipt sent back to transmitting router.
consequence: lots of bandwidth consumed and chaotic traffic

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 Solution to LSA flooding issue
is the use of
–Designated router (DR)
–Backup designated router (BDR)

 DR & BDR selection
–Routers are elected to send &
receive LSA

 Sending & Receiving LSA
–DRothers send LSAs via
multicast 224.0.0.6 to DR & BDR
–DR forward LSA via multicast
address 224.0.0.5 to all other
routers
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DR/BDR Election Process
 DR/BDR elections DO NOT occur in point to point
networks

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 DR/BDR elections will take place on multiaccess
networks as shown below

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 Criteria for getting elected DR/BDR
1. DR: Router with the highest OSPF
interface priority.
2. BDR: Router with the second highest
OSPF interface priority.
3. If OSPF interface priorities are equal, the
highest router ID is used to break the tie.

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 Timing of DR/BDR Election
–Occurs as soon as 1st router has its interface enabled on
multiaccess network
When a DR is elected it remains as the DR until one of the
following occurs
-The DR fails.
-The OSPF process on the DR fails.
-The multiaccess interface on the DR fails.

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 Manipulating the election process
-If you want to influence the election of DR & BDR then 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.

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OSPF Interface Priority
 Manipulating the DR/BDR election process continued
–Use the ip ospf priority interface command.
–Example:Router(config-if)#ip ospf priority {0 - 255}
Priority number range 0 to 255
–0 means the router cannot become the DR or BDR
–1 is the default priority value

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More OSPF Configuration
Redistributing an OSPF Default Route
 Topology includes a link to ISP
–Router connected to ISP
Called an autonomous system border router
Used to propagate a default route
–Example of static default route
R1(config)#ip route 0.0.0.0 0.0.0.0
loopback 1
–Requires the use of the default-information originate
command
–Example of default-information originate command
R1(config-router)#default-information
originate
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Fine-Tuning OSPF
 Since link speeds are getting
faster it may be necessary to
change reference bandwidth
values
–Do this using the auto-cost
reference-bandwidth
command
–Example:
 R1(config-router)#auto-cost
reference-bandwidth 10000

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Fine-Tuning OSPF
 Modifying OSPF timers
–Reason to modify timers
Faster detection of network failures
–Manually modifying Hello & Dead intervals
Router(config-if)#ip ospf hello-interval seconds
Router(config-if)#ip ospf dead-interval seconds
–Point to be made
Hello & Dead intervals must be the same between
neighbors

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Summary
 RFC 2328 describes OSPF link state concepts and
operations
 OSPF Characteristics
–A commonly deployed link state routing protocol
–Employs DRs & BDRs on multi-access networks
DRs & BDRs are elected
DR & BDRs are used to transmit and receive LSAs
–Uses 5 packet types:
1: HELLO
2: DATABASE DESCRIPTION
3: LINK STATE REQUEST
4: LINK STATE UPDATE
5: LINK STATE ACKNOWLEDGEMENT
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Summary
 OSPF Characteristics
–Metric = cost
Lowest cost = best path

 Configuration
–Enable OSPF on a router using the following command
R1(config)#router ospf process-id
–use the network command to define which interfaces will
participate in a given OSPF process
Router(config-router)#network network-address
wildcard-mask area area-id

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Summary
 Verifying OSPF configuration
–Use the following commands
show ip protocol
show ip route
show ip ospf interface
show ip ospf neighbor

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Exploration routing chapter_11

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