Cisco CCNA 2 Instructor Manual page 82

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:
Optional Labs:
Core TIs:
Optional TIs:
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
7.2.2, 7.2.6, 7.2.7, and 7.2.9
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