Unicast Routing Protocols

(RIP, OSPF, and BGP)

Chapter 11

2

Routing Protocol is combination of rules and

procedures lets routers in internet inform each

other of route changes

Autonomous System (AS) is group of networks

and routers under authority of single administration

Intradomain routing is routing inside AS

Handled by interior routing protocol

Interdomain routing is routing between AS’s

Handled by exterior routing protocol

Routing Protocols

3

Autonomous Systems

4

Routing Protocols

5

Distance Vector Routing

Uses Bellman-Ford algorithm for creating

routing table for routers in AS

Each node shares its routing table with its

immediate neighbors periodically and when

there is a change

6

Bellman-Ford Algorithm

7

Distance Vector Routing

8

Distance Vector Routing

Net4 , 1

3

Net5 , 1

4

Net2 , 1

2

9

Count to Infinity -- Two-Node Instability

10

Defining Infinity

Define 16 as infinity

Split Horizon

If node B reaches X via A, then B does not need

to advertise to A its (B's) distance to X

Split Horizon and Poison Reverse

If node B reaches X via A, then B will advertise

to A that its (B's) distance to X is infinity

Solutions to Instability

11

Three-Node Instability

12

Intradomain routing protocol

Based on distance vector routing

Uses hop count as metric (cost)

Infinite distance defined as 16 hops

Routing Information Protocol (RIP)

13

Example of a domain using RIP

14

RIP Message Format

Command: Request (1), Response (2)

Version: 1

Family: TCP/IP (2)

Request Message

Response (Update) Message

Solicited response: sent only in answer to request

Unsolicited response: sent periodically (every 30 sec)

15

Requests and Responses

16

Update Message – Example

Update message

from R1 to R2:

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RIP Timers

Periodic timer: How frequent to send update message

Expiration timer: How long to wait for update message

Garbage collection timer: How long to wait before purging route from table

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RIP Version 2

Route tag: AS number

Uses all-router multicast address to send RIP messages

Supports classless addressing

Message Format

19

RIP Version 2

Supports authentication

20

RIP messages encapsulated in UDP

datagrams

UDP uses well-known port (520) for RIP

Encapsulation

IP

header

UDP

header RIP message

20 bytes 8bytes

UDP datagram

IP datagram

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Link State Routing

Each node advertises its links state to all other

nodes (flooding)

Link state packet (LSP) carries node identity,

list of links, sequence number, and age

Each node uses same topology to create its

routing table

Each node constructs its routing table using

Dijkstra algorithm

22

Link State Routing

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Dijkstra Algorithm: Forming Shortest Path Tree

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Open Shortest Path First (OSPF) Protocol

Intradomain routing protocol

Based on link state routing

Metric can be based on type of service (min delay,

max throughput, …)

Divides autonomous system into areas

25

Types of Links

26

Types of Links

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Virtual Link

When link between two routers is broken, virtual link

created using path probably goes through several routers

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Types of OSPF Packet

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OSPF Common Header

Type: Defines the packet type (1 to 5)

Checksum: On entire packet excluding authentication

Authentication: None (0), Password (1)

64

(Router ID)

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Link State Update Packet

Used by router to advertise its links states

Each packet contains several different LSAs

(LSA)

31

Link State Update Packet

# of LSAs

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LSA General Header

Link State Type Link State ID

Type 1: Router link Address of router

Type 2: Network link Address of designated router

Type 3: Summary link to network Address of network

Type 4: Summary link to AS boundary router Address of AS boundary router

Type 5: External link Address of external network

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Router Link LSA

Announce all router links

34

Router Link LSA

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Router Link LSA Example

36

Router Link LSA Example

37

Network Link LSA

Announce network and all routers connected to it

Distributed by designated router on behalf of

transient network

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Network Link LSA

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Network Link LSA Example

0

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Network Link LSA Example

Router-link LSA

R1 has two links (N1, N2)

R2 has one link (N2)

R3 has two links (N2, N3)

Network-link LSA

N1 advertised by R1

N2 advertised by designated

router (R1, R2, or R3)

N3 advertised by R3

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Summary Link to Network LSA

Announce networks outside the area

Flooded by area boarder router

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Summary Link to Network LSA

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Summary Link to AS Boundary Router LSA

Announce AS boundary router

Flooded by area boarder router

44

Summary Link to AS Boundary Router LSA

45

External Link LSA

Announce networks outside AS

Flooded by AS boundary router

46

External Link LSA

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Hello Packet

To create neighborhood relationships

To test reachability of neighbors

Determine the selection of designated router

48

Database Description Packet

Sent by neighbors to new connected router

Contains outline (LSA header) of each link in

link-state database

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Link State Request Packet

Sent by router needs information about specific link

Can be used by newly connected router to request

more information after receiving database

description packet

50

Link State Acknowledgment Packet

To acknowledge receipt of every link state update

packet

51

OSPF packets encapsulated in IP datagrams

Encapsulation

IP

header OSPF packet

20 bytes

IP datagram

52

Path Vector Routing

Exterior routing protocol for inter-domain or inter-AS routing

Router has list of networks can be reached

with path (list of ASs to pass)

Prevents looping using path information

Allows routing based on router policy

Optimum path is one that fits organization

needs (security, safety, reliability, …)

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Reachability

54

Routing Tables

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Routing Tables After Aggregation

14 14 14

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Loop Prevention

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Border Gateway Protocol (BGP)

Interdomain routing protocol

Uses path vector routing

Supports classless addressing (CIDR)

Path is presented as list of attributes

58

Path Attributes

AS_PATH

Defines list of ASs to reach destination

NEXT-HOP

Defines next router to which packet sent

ORIGIN

Defines source of routing information (IGP: interior,

EGP: exterior)

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BGP Sessions

External BGP (E-BGP) session Exchange information between BGP routers

in different ASs

Internal BGP (I-BGP) session Exchange information between BGP routers

inside AS

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Types of BGP Messages

61

BGP Packet Header

Marker: reserved for authentication

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Open Message

To create neighborhood relationship

Neighbor responds with keepalive message

Hold time: waiting time until receiving keepalive or update message

BGP identifier: defines sending router (its IP address)

63

Update Message

To withdraw destinations advertised previously

To announce route to new destination

64

Network layer reachability information

Network to be advertised

(Length, Prefix) tuple Length: number of bits in prefix

Prefix: common part of network address

Example: IP address 153.18.7.0/24

Length: 24 Prefix: 153.18.7

Withdrawn routes

Destinations to be deleted from previously advertised list

(Length, Prefix) tuple

Update Message

65

Keepalive Message

Exchanged between peers regularly before hold

time expires

66

Notification Message

Sent by router whenever error detected or to close

connection

Error Code: Message header error (1), Open message error (2),

Update message error (3), Hold time expired (4).

67

BGP messages encapsulated in TCP segments

TCP uses well-known port (179) for BGP

Encapsulation