Cisco Systems CCNA 2 7.1. Distance Vector Routing

7.1. Distance Vector Routing

Essential Labs: None

Optional Labs: None

Core TIs: All

Optional TIs: none

Course-Level Claim: Students can identify, analyze, and show how to rectify inherent

problems associated with distance vector routing protocols.

Certification-Level Claim: Students can troubleshoot and configure routing protocols based

on user requirements.

Hands-on skills: none

Distance vector routing protocols require routers to forward their entire routing table when

passing along updates. Convergence is a step-by-step process with distance vector routing

protocols. This means that routing table information is forwarded to neighbor routers, which

continue to forward the information to their neighbors. This is contrasted with link-state routing

protocols, which forward their routing tables out to all routers in their area. These routing

tables include information about the total cost of a route and the logical address of the first

router on the path to each network contained in the table.

Routers need to update the information in their routing tables to continuously make good path

determination decisions. Periodically, changes in a network will affect the decisions made by a

router. For example, a router may be taken off line for upgrades or repairs or an interface on a

router may go down. If routers are not aware of the changes that have occurred in a network,

they may switch packets to interfaces that are no longer connected to the best route.

Distance vector routing protocols typically send out updates at certain time intervals such as

every 30 seconds for RIP. Sometimes distance vector routing protocols initiate updates when

topology changes occur. For example, IGRP sends out flash updates sooner than its standard

update interval of 90 seconds.

Routing loops can occur if slow convergence on a network causes inconsistent routing entries.

If a network goes down, this information may not get propagated across the network quickly

enough. As a result, a router may develop an incorrect view of the network and send out this

incorrect information.

Use the following example in class:

• Just before the failure of Network 1, all routers have consistent knowledge and

correct routing tables. The network is said to have converged. Assume for the

remainder of this example that for Router C, the preferred path to Network 1 is by

way of Router B and the distance from Router C to Network 1 is three.