Distance Vector Routing Protocols

Dynamic Routing

Objectives

Characteristics of Distance Vector Routing protocols.

Network discovery process using Routing Information Protocol (RIP).

Identify the conditions leading to a routing loop and explain the implications for router performance.

Review - Dynamic Routing Protocols

Function(s) of Dynamic Routing Protocols:Dynamically share

information between routers.

Automatically update routing table when topology changes.

Determine best path to a destination.

Interior Gateway Routing

Two Main ApproachesDistance Vector Protocols

E.g., RIP (Routing Information Protocol)

Link State ProtocolsE.g., OSPF (Open Shortest Path First)

Our FocusDistance Vector Protocols - RIP

Routing Protocols- Criteria to compare

Criteria used to compare routing protocols includes

Time to convergence Scalability Resource usage Implementation & maintenance

Routing Protocol Algorithm Algorithm - procedure for accomplishing a certain task

Distance Vector Routing Protocols

A router using distance vector routing protocols knows 2 things:

Distance to final destination

Vector, or direction, traffic should be directed

Distance Vector Routing Protocols

Characteristics Periodic updates Triggered updates Exchange

information with Neighbors

Entire routing table is included with routing update

Routing Table MaintenanceTriggered Updates

Conditions in which triggered updates are sentInterface changes

stateRoute becomes

unreachableRoute is placed in

routing table

Network Discovery Initial Exchange of Routing

InformationIf a routing protocol is configured

then routers will exchange routing information

Routing updates received from other routersRouter checks update for new

informationIf there is new information

Metric is updatedNew information is stored in

routing table

Routing Table - Periodic Updates

Regular Time Intervals in which a router sends out its entire routing table to its neighbours.

Routing Table Maintenance

RIP uses 4 timers-Update timer -Invalid timer -Holddown timer -Flush timer

ConvergenceExchange of Routing

InformationRouter convergence is

reached whenAll routing tables in the

network contain the same network information

Routers continue to exchange routing informationIf no new information is

found then Convergence is reached

RIP Commands

• Using router rip and network commands to enable RIP.

• Enabling RIP on an IP-addressed network.

• Monitoring IP packet flow using the show IP protocol command.

• The show ip route command.

Configuring RIP

The show ip protocols Command

Troubleshooting RIP Update Issues

• debug ip rip• show ip rip database • show ip protocols {summary} • show ip route • debug ip rip {events} • show ip interface brief

18

The Count-to-Infinity Problem

AA BB CC1 1

A's Routing Table B's Routing Table

C

to costvia(next hop)

2B C

to costvia(next hop)

1C

now link B-C goes down

C 2 C oo

C oo-C 2B

C oo C 3

C 3AC oo-

C 4 C oo

C oo-C 4B

• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”

• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B

• How can the Count-to-Infinity problem be solved?

Preventing Routing Updates through an Interface

Load Balancing with RIP