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Computer Networking

Today’s Lecture

• What is Routing

 Definition and Concepts

 Static Routing

 Dynamic Routing

• Routing Approaches

 Distance Vectore

 Link State

• Routing Protocols

 Autonomous Systems

 Interior Routing Protocols

 Exterior Routing Protocols

• RIP
• IGP

Routing ..

• How routers know each other?

 Each router when boots up

 Finds the networks to which it is attached

 Broadcast this information to other routers

 All routers behaves the same way and they come to know each

other after some time

• How routes are selected?

 Routers calculates routing paths using

 Routing tables

 Routing algorithm

• What happens when a router becomes unavailable?

 Routers periodically broadcast a message to ensure their presence

 Absence of a message from a router is considered a failure

 All routers updates their routing tables and no more packet is sent

to such a router

Routing Overview

Router A Router C

Router B

N

N

N

N

N

Routing Table

Routing Table

Routing Table

Autonomous Systems

AS An ISP’s Network

AS PIEAS

AS PTCL

IRP and ERP

• IRP

 Interior Routing Protocols

 Passes routing information between routers within AS

 Customized routing protocols may be implemented

 Protocol does not need to be implemented outside the system

 Provides detailed information about the system.

 Example

 RIP  Routing Information Protocol

ERP

 Exterior Routing Protocol

 Routers on borders of AS uses ERP

 ERP is used to provide summary information about AS routers

 Example

 EGRP  Exterior Gateway Routing Protocol

IRP and ERP…

Router with IRP and ERP both

Router with IRP and ERP both

Routing Approaches

• Static Routing

 Manual Configuration

 Routing updates are not possible

• Dynamic Routing

 Dynamic routing approaches are grouped as

 Distance Vector Routing

 Link State routing

 Both approaches uses different set of routing algorithm

Distance Vector Routing Protocols

• Routers gather information about directly attached networks

• Each router broadcast its routing tables

• Routing updates are periodic

 Each router broadcast its table after some specific period

 When a router receives an update from a router it starts a timer if

no update is received during this time the route is discarded

• Also called routing by rumor

 Routers broadcast information about the networks they are

directly attached along with network they heard form other routers

Distance Vector Routing…

• After some period all router come to know each other

 This period is called Network Convergence Time

• Distance between two networks is measured in terms of hop count

 Hop count  number of routers used

• Routing decisions are based on hop count
• Note:

 The network information is broadcasted only on directly connected

networks

• Routers A, B and C broadcast their routing tables/distance vectors
• When A receives some information from B and it updates its routing

table

A B^ C

Network 1 Network 2 Network 3 Network 4

Destination Distance

Network1 0

Network2 0

Network3 1 B

Networks accessible from Router B

Destination Distance

Network2 0

Network3 0

Updated Table of Router A

• B receives broadcast from A it will add network1 accessible from A
• B also receives a broadcast from C, it will add network4 accessible

from C

Destination Distance

Network1 1 A

Network2 0

Network3 0

Destination Distance

Network1 1 A

Network2 0

Network3 0

Network4 1 C

• Routing tables after some period [Network Convergence]

Destination Distance

Network1 0

Network2 0

Network3 1 B

Network4 2 B

Destination Distance

Network1 1 A

Network2 0

Network3 0

Network4 1 C

Destination Distance

Network1 2 B

Network2 1 B

Network3 0

Network4 0

Routing Table of Router A

Routing Table of Router B

Routing Table of Router C

Routing Loops

• Updates in distance vector routing are periodic

• If a link goes down the change is not propagated immediately

• Bad entries in routing table

A B^ C

Network 1 Network 2 Network 3 Network 4

Destination Distance

Network1 2 B

Network2 1 B

Network3 0

Link Failed Router C

Link Failed

Router C Removes The entry The change will be advertised after some period Other Routers still have this entry which is incorrect now Meanwhile Updates may arrive