Routing Protocols Essential for Network Engineer

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Routing Protocols (RIP,
OSPF, and BGP)

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Outline
 INTERIOR AND EXTERIOR ROUTING
 RIP
 OSPF
 BGP

Introduction
 An internet is a combination of networks connected
by routers
 How to pass a packet from source to destination ?
 Which of the available pathways is the optimum pathway ?
 Depends on the metric:
 Metric: a cost assigned for passing through a network
 A router should choose the route with the smallest metric

Introduction (Cont.)
 The metric assigned to each network depends on the
type of protocol
 RIP (Routing Information Protocol)
 Treat each network as equals
 The cost of passing through each network is the same: one hop
count
 Open Shortest Path First (OSPF)
 Allow administrator to assign a cost for passing through a
network based on the type of serviced required
 For example, maximum throughput or minimum delay
 Border Gateway Protocol (BGP)
 The criterion is the policy, which can be set by the administrator

Introduction (Cont.)
 Routing table can be static or dynamic
 An internet needs dynamic routing tables
 Dynamic routing table is achieved by the
routing prococols

INTERIOR
AND
EXTERIOR
ROUTING

Interior and Exterior Routing
 An internet can be so large that one routing
protocol cannot handle the task of updating
routing table of all routers
 Thus, an internet is divided into autonomous
systems (AS)
 AS is a group of networks and routers under the
authority of a single administration

Interior and Exterior Routing
 Interior routing
 Routing inside an autonomous system
 Each AS can chose its own interior routing
protocol
 Examples: RIP and OSPF
 Exterior routing
 Routing between autonomous systems
 Only one exterior routing protocol is usually used
for exterior routing
 Examples: BGP

Figure 13-1
Popular Routing Protocols

Example
 R1, R2, R3 and R4 use an interior and an
exterior routing protocol
 Solid thin lines
 Interior routing protocol
 Broken thick lines
 Exterior routing protocol

Figure 13-2
Autonomous Systems

RIP:
Routing
Information
Protocol

RIP
 RIP: Routing Information Protocol
 Based on distance vector routing
 Use the Bellman-Ford algorithm for calculating
the routing tables

Distance Vector Routing
 Each router periodically shares its knowledge
about the entire internet with its neighbors
 Sharing knowledge about the entire AS
 At the start, a router’s knowledge may be sparse
 But, how much it knows is unimportant, it sends
whatever it has
 Sharing only with neighbors
 Sends its knowledge only to neighbors
 Sharing at regular intervals

RIP Updating Information
 Routing table is updated on receipt of a RIP response message
 Receipt: a response RIP message
 Add one hop to the hop count for each advertised destination
 Repeat the following steps for each advertised destination
 If (destination not in the routing table)
 Add the advertised information to the table
 Else
 If (next-hop field is the same)
 Replace retry in the table with the advertised one
 Else
 If (advertised hop count smaller than one in the table)
 Replace entry in the routing table
 Return

Example of Updating a Routing Table

Initializing the Routing Table
 When a router is added to a network
 It initializes a routing table for itself using its
configuration file
 The table contains only the directly attached
networks and the hop count (= 1)

Updating the Routing Table
 Each routing table is updated upon receipt of
RIP message
 Using the RIP updating message algorithm shown
above

Initial Routing Tables in a Small
Autonomous System

Final Routing Tables for the
Previous Figure

RIP Message Format
 Command: 8-bit
 The type of message: request (1) or response (2)
 Version: 8-bit
 Define the RIP version
 Family: 16-bit
 Define the family of the protocol used
 TCP/IP: value is 2
 Address: 14 bytes
 Defines the address of the destination network
 14 bytes for this field to be applicable to any protocol
 However, IP currently uses only 4 bytes, the rest are all 0s
 Distance: 32-bit
 The hop count from the advertising router to the destination network

Figure 13-6
RIP Message Format

Requests and Response
 RIP uses two type of messages
 Request and response
 Request
 Sent by a router that has just come up or has some
time-out entries
 Can ask specific entries or all entries

Figure 13-7
Request Messages

Requests and Response (Cont.)
 Response: solicited or unsolicited
 A solicited response: sent only in answer to a
request
 Contain information about the destination specified in
the corresponding request
 An unsolicited response: sent periodically
 Every 30s
 Contains information about the entire routing table
 Also called update packet

 What is the periodic response sent by router
R1 in the next slide?
 Assume R1 knows about the whole
autonomous system.
Example 1

Figure 13-8
Example 1

 R1 can advertise three networks 144.2.7.0,
144.2.9.0, and 144.2.12.0.
 The periodic response (update packet) is
shown in the next slide.
Solution

Figure 13-9
Solution to Example 1

Timers in RIP
 RIP uses three timers
 Periodic timer
 Expiration timer
 Garbage collection timer

Figure 13-10
RIP Timers

Periodic Timer
 Periodic timer
 Control the advertising of regular update message
 Although protocol specifies 30 s, the working
model uses a random number between 25 and 35 s
 Prevent routers update simultaneously

Expiration Timer
 Govern the validity of a route
 Set to 180 s for a route when a router receives update
information for a route
 If a new update for the route is received, the timer is reset
 In normal operation, this occurs every 30 s
 If timer goes off, the route is considered expired
 The hop count of the route is set to 16, which means
destination is unreachable

Garbage Collection Timer
 When a route becomes invalid, the router does not
immediately purge that route from its table
 It continues advertise the route with a metric value of
16
 A garbage collection timer is set to 120 s for that
route
 When the count reaches zero, the route is purged from
the table
 Allow neighbors to become aware of the invalidity of
a route prior to purging

 A routing table has 20 entries.
 It does not receive information about five
routes for 200 seconds.
 How many timers are running at this time?
Example 2

 The timers are listed below:
 Periodic timer: 1
 Expiration timer: 20 - 5 = 15
 Garbage collection timer: 5
Solution

Problems in RIP
 Slow Convergence
 Instability

Slow Convergence
 A change somewhere in the internet
propagates very slowly through the rest of the
internet
 For example
 A change is Net1, R1 updates itself immediately
 R1->R2: an average of 15 s
 R2->R3: an average of 15 s
 …
 Thus, R1-Rn: an average of 15 x n s

Figure 13-11
Slow Convergence
0

Solution
 Limit the hop count to 15
 Prevent data packet from wandering around
forever
 Thus, an autonomous system using RIP is
limited to a diameter of 15
 The number 16 is considered infinity and
designates an unreachable network

Figure 13-12
Hop count

Instability
 An internet running RIP can become unstable
 A packet could go from one router to another in a
loop

Instability

Solutions
 Triggered Update
 Split Horizons
 Poison Reverses

Triggered Update
 If there is no change, updates are sent at the
30-s intervals
 If there is a change, the routers sends out its
new table immediately

Split Horizons
 A router must distinguish between different interface
 If a router received route updating message from an
interface
 This same updated information must not be sent back
through this interface
 Example
 B receives information about Net1 and Net2 through its
left interface
 This information is updated and passed on through the
right interface but not to the left

Figure 13-14
Split Horizon

Poison Reverse
 A variation of split horizons
 Information received is used to update routing table
and then passed out to all interface
 However, a table entry that has come through one
interface is set to a metric of 16 as it goes out
through the same interface
 For example
 Router B has received information about Net1 and Net2
through its left interface
 Thus, it sends information out about Net1 and Net2 with a
metric of 16

Figure 13-15
Poison Reverse

RIP Version 2
 Does not augment the length of the message
of each entry
 Only replace those fields in version 1 that
were filled with 0s with some new fields

RIP Version 2
 New fields
 Routing Tag: carries information such as the
autonomous system number
 Enable RIP to receive information from an exterior
routing table
 Subnet mask: carries the subnet mask (or prefix)
 RIP2 support classless addressing and CIDR
 Next-hop address: show the address of the next
hop

Figure 13-16
RIP-v2 Format

RIP version 2 supports
CIDR.

Authentication
 Protect the message against unauthorized
advertisement
 The first entry of the message is set aside for
authentication information
 Family field = FFFF16

Figure 13-17
Authentication

Multicasting
 Version 1 of RIP uses broadcasting to send
RIP message to every neighbor
 All the router and the hosts receive the packets
 RIP version 2
 Uses the multicast address 224.0.0.9 to multicast
RIP message only to RIP routers in the network

Encapsulation
 RIP message are encapsulated in UDP user
datagram
 The well-known port assigned to RIP in UDP
is port 520

RIP uses the services of UDP
on well-known port 520.

OSPF:
Open Shortest
Path First

OSPF
 OSFP: Open Shortest Path First
 Another interior routing protocol
 OSPF divides an autonomous system into
areas
 To handle routing efficiently and in a timely
manner

Areas
 A collection of networks, hosts, and routers
all contained within an autonomous system
 Thus, an autonomous system can be divided
into many different areas
 All networks inside an area must be
connected

Areas (Cont.)
 Routers inside an area flood the area with
routing information
 Each area has a special router called area
border routers
 Summarize the information about the area and
sent it to other areas

Areas (Cont.)
 Among the area inside an autonomous system
is a special area called backbone
 All of the areas inside an AS must be connected
to the backbone
 The routers inside the backbone are called the
backbone routers
 A backbone router can also be an area border
router

Areas (Cont.)
 If the connectivity between a backbone and an
area is broken
 A virtual link must be created by the
administration
 Each area has an area identification
 The area identification of the backbone is zero

Figure 13-18
Areas in an Autonomous System

Metrics
 OSPF allows the administrator to assign a
cost, called the metric, to each route
 Metric can be based on a type of service
 Minimum delay
 Maximum throughput
 A router can have multiple routing tables
 Each based on a different type of service

Link State Routing
 OSPF uses link state routing to update the
routing table in an area
 Link state routing: a process by which each
router shares its knowledge about its neighbor
with every router in the area

Link State Routing (Cont.)
 Three keys
 Sharing knowledge about the neighborhood
 Each router sends the state of its neighborhood to
every other router in the area
 Sharing with every other router
 Each router sends the state of its neighborhood to
every other router in the area by flooding
 Sharing when there is a change
 Each router shares the state of its neighborhood only
when there is a change

Link State Routing (Cont.)
 The idea behind link state routing
 Each router should have the exact topology of the
internet at every moment
 Thus, every router should have the whole
picture of the internet
 A router can calculate the shortest path between
itself and each network
 But, how to represent an internet by graph?

Types of Links
 In OSPF, a connection is called a link
 Four types of links
 Point-to-point
 Transient
 Stub
 Virtual

Figure 13-19
Types of Links

Point-to-Point Link
 Connect two routers without any other host or router
in between
 Example
 Telephone line
 T-line
 Graphically representation
 The routers are represented by nodes
 The link is represented by a bidirectional edge
 The metric
 Usually the same at the two ends

Figure 13-20
Point-to-Point Link

Transient Link
 A network with several routers attached to it
 Data can enter through any of the routers and
leave through any router
 Example
 All LANs and some WANs with two or more
routers

Transient Link (Cont.)
 Figure b in the next slide. However, it is
 Not efficient: each router need to advertise the neighborhood of
four other routers
 For a total of 20 advertisement
 Not realistic: there is no single network (link) between each pair
of routers
 There should be only one network that serves as a crossroad
between all five routers
 Solution: one of the routers acts as a single network
 This router has a dual purpose: a true router and a
designated router
 The link is represented as a bidirectional edge
 Figure c in the next slide

Figure 13-21
Transient Link

Stub Link
 A network that is connected to only one router
 Data packet enter and leave through this only one
router
 A special case of transient network
 The router as a node
 The designed router as the network
 Note, the link is only one-directional
 From the router to the network

Figure 13-22
Stub Link

Virtual Link
 When the link between two routers is broken
 The administrator may create a virtual path
between them using a longer path and may go
through several routers

Figure 13-23
Example of an internet

Figure 13-24
Graphical Representation of an internet

Link State Advertisements
 Each entity distributes Link State Advertisement to
share information about their neighbors
 An LSA announces the states of entity links
 Five LSAs, depend on the type of entity
 Router link
 Network link
 Summary link to network
 Summary link to AS boundary router
 External link

Figure 13-25
Types of LSAs

Router Link
 Define the links of a true router
 A true router uses this advertisement to
announce information about
 All of its links
 What is at the other side of the links (neighbors)

Figure 13-26
Router Link

Network Link
 Defines the links of a network
 A designated router, on behalf of the transient
network, distributes this type of LSA packet
 Announce the existence of all of the routers
connected to the network

Figure 13-27
Network Link

Summary Link to Network
 Router link and network link advertisements
 Flood the area with information inside an area
 But a router must also know about the networks
outside its area
 The area border routers provide this information
 An area border is active in more than one area
 Receive router link and network link advertisements
 Create a router table for each area
 Provide one area’s information to other areas by the
summary link to network advertisement

Example
 R1 is an area border router
 Two routing table: one for area 1 and one for area
0
 R1 will flood area 1 with information about
how to reach a network located in area 0
 R2 plays the same role

Figure 13-28
Summary Link to Network

Summary Link to AS Boundary Router
 Previous advertisement lets every router know the
cost to reach all of the networks inside the AS
 But, how to reach a network outside an AS?
 A router must know how to reach the autonomous
boundary router first
 The summary link to AS boundary router provides
this information
 The area border routers flood their area with this
information

Figure 13-29

External Link
 How a router inside an AS know which
networks are available outside the AS ?
 The AS boundary routers floods the
autonomous system with the cost of each
network outside the AS
 Using a routing table created by an exterior
routing protocol

External Link (Cont.)
 Notably, each advertisement announces one
single network
 Separate announcements are made if more than
one network exists

Figure 13-30
External Link

 In Figure 13.31 (next slide), which router(s)
sends out router link LSAs?
Example 3

Figure 13-31
Example 3 and Example 4

Solution
All routers advertise router link LSAs.
R1 has two links, Net1 and Net2.
R2 has one link, Net2 in this AS.
R3 has two links, Net2 and Net3.

Example 4
In Figure 13.31, which router(s) sends out
the network link LSAs?

Solution
All three network must advertise network links:
Advertisement for Net1 is done by R1 because it is
the only router and therefore the designated router.
Advertisement for Net2 can be done by either R1,
R2, or R3, depending on which one is chosen as
the designated router.
Advertisement for Net3 is done by R3 because it is
the only router and therefore the designated router.

Link State Database
 Every router in an area
 Receive the router link and network link LSA
from every other router
 And form a link state database
 A tabular representation of the topology of the internet
inside an area
 Notably, every router in the same area has the
same link state database

In OSPF, all routers have
the same link state database.

Dijkstra Algorithm
 Each router applies the Dijkstra algorithm to
from its link state database
 Dijkstra algorithm
 Calculate the shortest path between two points

F
igure 13-32-Part 1
Shortest Path Calculation

F
igure 13-32-Part 2

F
igure 13-32 Part 3

Routing Table
 Each routers uses the shortest path tree method to
construct its routing table
 Routing table in OSPF includes the cost of reaching
each network in the area
 To find the cost of reaching network outside of the
area, the routers use the
 Summary link to network
 Summary link to boundary router
 External link advertisements

Types of Packets
 OSPF uses five different packets
 Hello packet
 Database description packet
 Link state request packet
 Link state update packet
 Router link
 Network link
 Summary link to network
 Summary link to AS boundary router
 External link
 Link state acknowledgment packet

Figure 13-33
Types of OSPF Packets

Packet Format
 All OSPF packets share the same header
 Version: 8-bit
 The version of the OSPF protocol. Currently, it is 2
 Type: 8-bit
 The type of the packet
 Message length: 16-bit
 The length of the total message including the header
 Source route IP address: 32-bit
 The IP address of the router that sends the packet

Figure 13-34
OSPF Packet Header

Packet Format (Cont.)
 Area identification: 32-bit
 The area within which the routing take place
 Checksum: 16-bit
 Error detection on the entire packet excluding the authentication
type and authentication data field
 Authentication type: 16-bit
 Define the authentication method used in this area
 0: none, 1: password
 Authentication: 64-bit
 The actual value of the authentication data
 Filled with 0 if type = 0; eight-character password if type = 1

Hello Message
 OSPF uses the hello message to
 Create neighborhood relationships
 Test the reachability of neighbors
 First step in link state routing
 It must first greet its neighbors

Figure 13-35
Hello Packet

Hello Packet Format
 Network mask: 32-bit
 Define the network mask of the network over which the
hello message is sent
 Hello interval: 16-bit
 Define the number of seconds between hello message
 E flag: 1-bit
 If it is set, the area is a stub area
 F flag: 1-bit
 If it is set, the router supports multiple metrics

Hello Packet Format (Cont.)
 Priority
 The priority of the router. Used for the selection
of the designated router
 The router with the highest priority is chosen as
the designated router
 The router with the second highest priority is
chosen as the backup designated router
 If it is 0, the router never wants to be a designated
or backup designated router

Hello Packet Format (Cont.)
 Dead interval: 32-bit
 The number of seconds before a router assumes
that a neighbor is dead
 Designated router IP address: 32-bit
 Backup designated router IP address: 32-bit
 Neighbor IP address: a repeated 32-bit field
 A current list of all the neighbors from which the
sending router has received the hello message

Database Description Message
 When a router is connected to the system for
the first time or after a failure
 It needs the complete link state database
immediately
 Thus, it sends hello packets to greet its
neighbors
 If this is the first time that the neighbors hear
from the router
 They send a database description packet

Database Description Message (Cont.)
 The database description message does not contain
complete database information
 It only gives an outline, the title of each line in the
database
 The newly router examines the outline and find out
which lines it does not have
 Send one or more link state request packets to get full
information about that particular link
 The content of the database may be divided into several
message

Database Description Message (Cont.)
 When two routers want to exchange database
description packets
 One of them acts as mater
 The other is the slave

Figure 13-36
Database Description Packet

Database Description Message Format
 E flag: 1-bit
 Set to 1 if the advertising router is an autonomous
boundary router
 B flag: 1-bit
 Set to 1 if the advertising router is an area border router
 I flag: 1-bit, the initialization flag
 Set to 1 if the message is the first message
 M flag: 1-bit, more flag
 Set to 1 if this is not the last message

Database Description Message Format
(Cont.)
 M/S flag: 1-bit, master/slave flag
 Indicate the origin of the packet. Master = 1, Slave = 0
 Message sequence number: 32-bit
 Contain the sequence number of the message
 LSA header: 20-bit
 Used in each LSA
 The format of this header is discussed in the link state
update message
 Only give the outline of each link
 It is repeated for each link in the link state database

Link State Request Packet
 Sent by a router that needs information about
a specific route or routes
 Answered with a link state update packet
 Used by a newly connected router to request
more information after receiving the database
description packet

Figure 13-37
Link State Request Packet

Link State Update Packet
 Used by a router to advertise the state of its
links
 Each update packet may contain several
different LSAs
 Packet format
 Number of link state advertisements: 32-bit
 Link state advertisement
 There are five different LSAs, as discussed before
 All have the same header format, but different bodies

Figure 13-38
Link State Update Packet

LSA Format
 Link state age: the number of seconds elapsed
since this message was first generated
 LSA goes from router to router, i.e., flooding
 When a router create a message, age = 0
 When each successive router forwards this message
 Estimate the transmit time and add it to the age field
 E flag: if 1, the area is a stub area
 i.e., an area that is connected to the backbone area
by only one path

LSA Format (Cont.)
 T flag: if 1, the router can handle multiple
types of service
 Link state type
 1: router link
 2: network link
 3: summary link to network
 4: summary link to AS boundary router
 5: external link

LSA Format (Cont.)
 Link state ID: depend on the type of link
 Router link: IP address of the router
 Network link: IP address of the designed router
 Summary link to network: address of the network
 Summary link to AS boundary router: IP address
of the AS boundary router
 External link: address of the external network

LSA Format (Cont.)
 Advertisement router:
 IP address of the router advertising this message
 Link state sequence number:
 Sequence number assigned to each link state update
message
 Link state checksum:
 A special checksum algorithm: Fletcher’s checksum
 Length:
 Total packet length

Figure 13-39
LSA Header

Router Link LSA
 Advertise all of the links of a router
 Format
 Link ID:
 Depend on the type of link, see Table 13.3
 Link data:
 Give additional information about the link, also depend on the
type of link, see Table 13.3
 Link type:
 Four different types of links are defined based on the type of
network, see Table 13.3
 Number of types of services (TOS)
 The number of type of services announced for each link

Router Link LSA (Cont.)
 Metric for TOS 0:
 Define the metrics for the default type of service
(TOS 0)
 TOS:
 Define the type of service
 Metric:
 Define the metric for the corresponding TOS

Figure 13-40
Router Link LSA

Table 13.3
Link Type Link Identification Link Data
Type 1: point-to-
point connection to
another network
Address of neighbor
router
Interface number
Type 2: connection
to any-to-any
network
Address of designed
router
Router address
Type 3: connection
to stub network
Network address Network mask
Type 4: virtual link Address of neighbor
router
Router address

Example 5
Give the router link LSA sent by router
10.24.7.9 in Figure 13.41.

Figure 13-41
Example 5

 This router has three links
 Two of type 1 (point-to-point)
 One of type 3 (stub network)
 Figure 13.42 shows the router link LSA
Solution

Figure 13-42
Solution to Example 5

Network Link LSA
 Announce the links connected to a network
 Format
 Network mask
 Define the network mask
 Attached router
 Define the IP addresses of all attached routers

Figure 13-43
Network Link Advertisement Format

Summary Link to Network LSA
 Used by the area router to announce the existence of
other networks outside the area
 Each advertisement announces only one network
 If more than one network, a separate advertisement must
be issued for each
 Format
 Network mask
 TOS:
 Type of service
 Metric:
 Metric for the type of service defined in the TOS field

Figure 13-46
Summary Link to Network LSA
The McGraw-Hill Companies, Inc., 2000

LSA
 Announce the route to an AS boundary router
 Define the network to which the AS boundary
router is attached
 Format
 The same as the summary link to network LSA

Figure 13-47
Summary Link to AS Boundary LSA

External Link LSA
 Announce all the networks outside the AS
 Format: similar to the summary link to AS
boundary router LSA but add two fields
 Forwarding address
 Define a forward router than can provide a better
route to the destination
 External route tag
 Used by other protocol, but not by OSPF

Figure 13-48
External Link LSA

Link State Acknowledgment Packet
 OSPF forces every router to acknowledge the
receipt of every link state update packet
 Make routing more reliable
 Format
 Common header
 Link state header

Figure 13-49
Link State Acknowledgment Packet

Encapsulation
 OSPF packets are encapsulated in IP
datagram
 OSPF contains the acknowledgment mechanism
for flow and error control
 Doe not need a transport layer protocol to provide
these services

OSPF packets are
encapsulated in IP datagrams.

BGP:
Border Gateway
Protocol

BGP
 BGP: Border Gateway Protocol
 An inter-autonomous system routing protocol
 Based on the path vector routing method

BGP (Cont.)
 Why the distance vector routing and link state
routing are not good candidates for inter-
autonomous system routing?
 Distance vector routing
 There are occasions in which the route with the smallest
hop count is not the preferred route
 For example, we may not want a packet to pass through an AS
that is not secure even it is the shortest path
 Unstable as discussed before

BGP (Cont.)
 Link state routing
 An internet is usually too big for this routing
method
 If used, each router must have a huge link state
database
 It would take a long time for each router to calculate
its routing table by Dijkstra algorithm

Path Vector Routing
 Each entry in the routing table contains
 The destination network
 The next router
 The path to reach the destination, usually defined
as ordered list of ASs

Example of a Path Vector Routing
Table
Network Next Router Path
NO1 R01 AS14, AS23, AS67
NO2 R05 AS22, AS67, AS05,
AS89
NO3 RO6 AS67, AS89, AS09,
AS34
NO4 R12 AS62, AS02, AS09

Path Vector Message
 Autonomous boundary routers advertise the
reachability of networks in their own AS to neighbor
autonomous boundary routers
 Each router receives a path vector message
 Verify the advertised path is in agreement with its policy
 If yes, update its routing table and modify the message
before sending it to the next router
 Add its AS number to the path and replacing the next router entry
with its own identification

Example
 R1 send a path vector message
 Advertise the reachability of N1
 R2 receives the message
 Update it routing table
 Send the message to R3 with adding its AS to the
path and inserting itself as the next router

Figure 13-50
Path Vector Packets
The McGraw-Hill Companies, Inc., 2000

Loop Prevention
 Path vector routing can solve
 The instability of distance vector routing
 The creation of loops
 When a router receive a message, it check to see if its
AS is in the path list or not
 If yes, looping is involved and the message is ignored

Policy Routing
 Policy routing can be easily implemented through
path vector routing
 Once a router receives a message, it can check the path.
 If one of the AS listed in the path is against its policy,
 It can ignore that path and that destination
 Does not update its routing table with this path
 Does not send this message to its neighbors
 Thus, path vector routing are not based on the
smallest hop count or the minimum metric
 Based on the policy imposed on the router by the
administrator

Path Attributes
 In previous example, the path was presented
as a list of AS
 Actually, the path was presented as a list of
attributes
 The list of attributes help the receiving router
make a better decision when applying its policy

Path Attributes (Cont.)
 Attributes are divided into two categories: well-known and
optional
 Well-known: one that every BGP router should recognize
 Mandatory
 Must appear in the description of a route
 e.g., ORIGIN: the source of the routing information (RIP or OSPF)
 e.g., AS_PATH: the list of AS through which the destination can be
reached
 e.g., NEXT_HOP: the next router to which data packet should be sent
 Discretionary
 Must be recognized by each router
 But is not required to be included in every update message

Path Attributes (Cont.)
 Optional: one that need not be recognized by
every router
 Transitive
 Must be passed to the next router by the router that
has not implemented this attribute
 Nontransitive
 One that should be discarded if the receiving router
has not implemented it

Types of Packets
 BGP uses four different types of messages
 Open
 Update
 Keepalive
 Notification

Figure 13-51
Types of BGP Messages

Packet Format
 All BGP packets share the same common
header
 Header format
 Marker: 16-bit
 Reserved for authentication
 Length: 2-bytes
 Define the length of the total message, including the
header
 Type: 1-byte
 Define the type of the packet

Figure 13-52
BGP Packet Header

Open Message
 Used to create a neighborhood relationship
 A router running BGP opens a TCP
connection with a neighbor and sends an open
message
 If the neighbor accepts
 It responses with a keepalive message
 The relationship then has been established
between the two router

Figure 13-53
Open Message

Open Message Packet Format
 Version: 1-byte
 Define the version of BGP. The current version is 4
 My autonomous system: 2-byte
 Define the autonomous system number
 Hold time: 2-byte
 Define the maximum number of seconds that can elapsed
before one of the parties receives a keepalive or update
message from the other
 If a router does not receive one of the messages during the
hold period, it considers the other party dead

Open Message Packet Format (Cont.)
 BGP number: 4-byte
 Define the router that sends the open message
 Option parameter length: 1-byte
 Define the length of the total option parameters
 Since open message may also contain some option
parameters
 Option parameters
 The only option parameters defined so far is
authentication

Update Message
 Used by a router to
 Withdraw destination that have advertised
previously
 Announce a route to a new destination
 Note, a router can withdraw several
destinations in a single update message
 However, it can only advertise one new
destination in a single update message

Update Message Format
 Unfeasible routes length: 2-byte
 Define the length of the next field
 Withdraw routes
 List all routes that should be deleted from the previously
advertised list
 Path attributes length: 2-byte
 Define the length of the next field
 Path attributes:
 Defines the attributes of the path (route) to the network
whose reachability is being announced in this msssage

Figure 13-54
Update Message

Update Message Format
 Network Layer reachability information
(NLRI)
 Define the network that is actually advertised by
this message
 Has two fields
 Length: define the number of bits in the prefix
 IP address prefix:
 Thus, BGP4 supports classless addressing and CIDR

BGP supports classless
addressing and CIDR.

Keepalive Message
 The BGP routers exchange keepalive message
regularly
 Tell each other that they are alive
 Format
 Consist of only the common header

Figure 13-55
Keepalive Message

Notification Message
 Sent by a router
 Whenever an error condition is detected
 A router wants to close the connection
 Format
 Error code: 1-byte, define the category of the error
 Message header error
 Open message error
 Update message error
 Hold timer expired
 Finite state machine error
 Cease
 Error subcode: 1-byte
 Furthermore define the type of error in each category
 Error data
 Used to give more diagnostic information about the error

Figure 13-56
Notification Message

Encapsulation
 BGP message are encapsulated in TCP
segments using the well-known port 179
 No need for error control and flow control
 Thus, when a TCP connection is opened
 The exchange of update, keepalive, and
notification message is continued
 Until a notification message of type cease is sent

BGP uses the
services of TCP on port 179.

Routing Protocols Essential for Network Engineer

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