[c-nsp] BGP Cease notifications with Graceful Restart

[John Neiberger]

We ran into an interesting issue recently and I'm not sure what to think of
it. We have a 7600 and a CRS peering via eBGP. The session was shutdown on
the 7600, which the CRS immediately recognized, but the CRS continued to
use those BGP routes until the neighbor's graceful restart timer expired.
The default for that timer is 120 seconds. RFC 4724 says that if a neighbor
notifies a peer that it is restarting, routes learned from the restarting
router should be marked as stale, but they must not be treated differently
than other routes in the BGP table. That makes sense if the router receives
a notification that the peer is restarting, but why does that make any
sense at all if it receives a notification that the session as been
administratively shutdown?

I contend that it makes more sense to immediately purge those routes from
the BGP table instead of continuing to use them until the restart timer
expires. This seems to be how the 7600 behaves if we shut the session down
from the CRS. But if we shut the session down from the 7600, the CRS does
not purge the routes for two minutes.

What are your thoughts on this? What sort of behavior should we be
expecting?

Thanks,
John