[e2e] iBGP session break

Zartash Afzal Uzmi zartash at lums.edu.pk
Mon Apr 16 03:09:36 PDT 2007

>>Greetings everyone
>>My question is: Is it possible for an iBGP session to break even when the
underlying IP topology is not partitioned? 
>>We would expect that IGP maintains connectivity at the IP layer, and even
in case of a failure in the network that doesnt partition the topology, the
>>IGP should be able to quickly converge to a new configuration for paths
between every possible pair of nodes. This should result in, perhaps, a few
>>dropped TCP packets for the iBGP session, but not a session break. Would
it, then still be possible for an iBGP session to break and blackholes to
>>consequently result? 
>It is certainly possible depending on the configuration.  BGP uses hold and
>timers.  If a session has its hold timer expire because of not receiving a
keepalive, then 
>the session will be terminated, even though TCP would not normally have
torn down 
>the connection on its own. 
>In the typical case (hold timer 180 sec, keep alive timer 60 sec), you
>wouln't lose the peering while the IGP is reconverging.  But if these
>are set very low, then you might.

I wonder of there exist scenarios (in real world configurations, maybe!)
when BGP timers are set at such low values that BGP session breaks while IGP
is still re-converging? If setting BGP timers to such low values is not very
common, is it safe to assume that a break in BGP session is also not very
common? Are there other reasons (other than IGP convergence taking longer
than BGP timers) which may cause BGP sessions to break?


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