Cisco Systems CCNA 2 manual Problem Routing Loops

•When Network 1 fails, Router E sends an update to Router A. Router A stops routing packets to Network 1, but Routers B, C, and D continue to route packets because they have not yet been informed of the failure. When Router A sends out its update, Routers B and D stop routing to Network 1. However, Router C has not received an update. Router C still tries to reach Network 1 through Router B.

•Now Router C sends a periodic update to Router D, which indicates a path to Network 1 through Router B. Router D changes its routing table to reflect this incorrect information and propagates the information to Router A. Router A propagates the information to Routers B and E and the process continues. Any packet that is destined for Network 1 will now loop from Router C to B to A to D and back again to C.

Problem: Routing Loops

Convergence is when all routers have the same information about the network. Convergence is a by-product of the routing updates that are sent out based on the routing protocol used on a router. If updated information does not reach all routers in a network quickly enough, then incorrect routing information may be sent out by routers that have not received the updates, which will replace the correct information in other routers.

In the example, Router C sends out an update to neighbor routers that incorrectly indicates that a route to Network 1 exists. This is a timing issue. Router C sends out updates before its neighbors have a chance to send out their newly updated information. Therefore, the accurate information is replaced by inaccurate information, which creates a routing loop.

A kinesthetic activity may be helpful to students to get a mental picture of how this process occurs. Instruct students to write their updates on paper and reenact the scenario from the figure and description.

77 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 7 Copyright ♥ 2004, Cisco Systems, Inc.