[c-nsp] IOS XR: routes with unreachable next-hops staying installed until BGP times out

[Blake Dunlap]

I don't think you've given all the details so it's hard to say, but do you
have graceful-restart configured in bgp / NSF?

-Blake


On Wed, Aug 7, 2013 at 1:27 PM, Drew Weaver <drew.weaver at thenap.com> wrote:

> Greetings,
>
> I am configuring BGP on an IOS XR 4.2.1 system and I have noticed that
> even though the router immediately determines that a next hop is
> unreachable it keeps the routes installed until BGP times out.
>
> Under normal circumstances 'show bgp nexthop ip.add.re.ss' looks like this:
>
> RIB Related Information
>     Gateway: reachable, non-Connected route, prefix length 32
>
> When there is a connectivity failure between the two systems it almost
> immediately changes to this:
>
> RIB Related Information
>     Gateway: unreachable, non-Connected route, prefix length 4
>
> Why then, given that it knows it's no longer a valid nexthop would it
> continue to route prefixes to it?
>
> Routing entry for 192.168.116.0/24
>   Known via "bgp xxxxx", distance 200, metric 0, type internal
>   Installed Aug  7 22:08:15.809 for 00:12:09
>
> It also continues to announce this route to upstream peers; again even
> though it knows it can't reach the next hop.
>
> It does eventually stop doing this; but only after the normal BGP timeout
> period (which I believe is 300 seconds).
>
> Is there a setting you need to enable so that if there is a RIB failure it
> will immediately stop doing this?
>
> Thanks,
> -Drew
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