7.1.7 Preventing routing loops with holddown timers

Holddown timers are used to prevent update messages from reinstating inaccessible routes. When a router receives an update that indicates that a network is unreachable, it starts a hold- down timer. While the hold-down timer is running, the router will not accept any updates about the inaccessible route unless the update comes from the originator of the triggered update or from a router reporting a better metric to the inaccessible network.

If a router receives routing update information from a router other than the originator of the triggered update that says it has a route to the inaccessible network with a lower metric than the original metric, the router ignores the update information while the holddown timer is still in effect.

Holddown timers are used to allow updates about bad routes to be propagated. Routers that have already received the information will not accept update information about the bad route from neighbor routers that may not know that it is inaccessible.

Students may need some additional help with distance vector routing protocols. Several of the topics will be discussed later in the RLO. It may be helpful to discuss related concepts such as holddown timers, route poisoning, and triggered updates in a combined lesson with the entire class. Group discussions about how these features fit together to help prevent routing loops may be helpful to students.

7.2 RIP

Essential Labs:

7.2.2, 7.2.6, 7.2.7, and 7.2.9

Optional Labs:

None

Core TIs:

All

Optional TIs:

none

Course-Level Claim: Students can configure, verify, analyze, and troubleshoot simple distance vector routing protocols.

Certification Level Claim: Students can troubleshoot and configure routing protocols based on user requirements.

Hands-on skills: none

81 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 7

Copyright 2004, Cisco Systems, Inc.

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Cisco Systems CCNA 2 manual Rip, Preventing routing loops with holddown timers