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

[Drew Weaver]

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