[c-nsp] eBGP multihop, link failure, and multi-path IGP (OSPF)

[David Prall]

Turn on PIC-Core
cef table output-chain build favor convergence-speed ! please be wary of
platform specific caveats

ip routing protocol purge interface ! purges interface routes and not routes
that followed the interface, this will leave the BGP routes untouched.

This is the only thing I could find discussing it:
http://www.cisco.com/en/US/docs/routers/10000/10008/configuration/guides/bro
adband/dffsrv.html#wp1191135

It is available on other platforms as well.

David

--
http://dcp.dcptech.com
 
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-
> bounces at puck.nether.net] On Behalf Of Rick Ernst
> Sent: Monday, November 02, 2009 4:04 PM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] eBGP multihop, link failure, and multi-path IGP (OSPF)
> 
> We have some eBGP neighbors that have their peering session reset in
> the
> case of link failure (root-cause analysis and problem resolution as a
> separate subject).  The peers are connected via loopback interfaces and
> multi-path OSPF.
> 
> bgp fast-external-failover is supposed to be used for directly
> connected
> eBGP peers, but it seems like a link failure on a pair of redundant
> (layer-3) links is also causing the peer to go down:
> Nov  1 11:33:12 10.56.205.1 %OSPF-5-ADJCHG: Process 1, Nbr a.b.c.d on
> FastEthernet8/0/0 from EXSTART to DOWN, Neighbor Down: Interface down
> or
> detached
> Nov  1 11:33:12 10.56.205.1 %BGP-5-ADJCHANGE: neighbor w.x.y.z Down
> Interface flap
> 
> The destination to the peer is still in the FIB, and the peer comes
> back up
> almost immediately (in this case, about 15 seconds).
> 
> I'm considering disabling fast-external-failover, but want to better
> understand the event.  The eBGP peer is not "directly connected" on the
> interface. It is reachable via a loopback peering IP with multi-path
> OSPF.
> Is this expected behavior (any link with a route to the destination
> going
> down will cause the session to go down)?
> 
> 
> Any gotchas with disabling fast-failover?
> 
> Thanks,
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