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