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Count to Infinity Problem With Distance-Vector Routing Protocols

Posted on 2011-03-08
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Hi,

I'm revising for my Cisco CCNA. I understand the general concept of count-to-infinity, but I have a problem with it: It was my understanding that routing tables only updated themselves if they were sent updates with routes that were shorter than that already stored. With the count-to-infinity scenario, the routing metric will always be bigger than that currently stored in the table, so the router shouldn't update the routing table to include it. How then, does the metric ever rise to infinity? And if this isn't the case (i.e. routers update regardless of metric values), surely the routing table can't be described as "a table containing the shortest known paths to different networks"?

Chris
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Question by:chris_smith_51
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by:Don Johnston
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>How then, does the metric ever rise to infinity?

Because the route with the shorter metric ages out. Once that happens the route with the higher metric is now accepted.

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by:Paul_McClure
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Think of the scenario with routers A B and C connected as A-B-C. If router A goes down then router B does not receive any packets. Router C on the other hand will tell router B that router A is only 2 hops away. Router B will assume that router A is available from router C and it will slowly propagate through the network. The split-horizon method can reduce the probability of a count-to-infinity problem but in nearly all cases a hold timer within a routing protocol can remove the possibility of this happening.
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by:chris_smith_51
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donjohnston> That ties in with something I briefly read. So packets enter a routing loop until the shorter metric gets aged out, at which point it is replaced by the higher metric and so on...? What determines the age-out time? Is it configurable or dependent on network management protocols? Typically, what would this time be?

Cheers
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Don Johnston earned 250 total points
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>What determines the age-out time?

The routing protocol. For example, in RIP it's 180 seconds. And like everything, it's adjustable.



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