Cisco Systems CCNA 2 manual Distance Vector Routing, Distance vector routing updates

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

7.1.1 Distance vector routing updates

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.

7.1.2 Distance vector routing loop issues

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.

76 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 7 Copyright ♥ 2004, Cisco Systems, Inc.