Send BGP update immediately if EIGRP route is missing

[esa_fresa]

We have two routers connected over an MPLS / BGP link, one at our primary datacenter and the other router at our backup datacenter. We have a route redistributed on both sides from EIGRP into both BGP routers. We changed the Administrative Distance on the primary router to be better than eBGP (set the AD to 19) so that it's used instead of the BGP route.

The issue is when the EIGRP route drops at the primary location, I don't think BGP will update as fast, so we'll have a temporary routing loop. Ie. The primary router sees its best route as over the MPLS link and sends traffic that way, but the neighbor router at the secondary location doesn't know that the primary router lost its route yet, so it sends the traffic back to the primary router. So my questions are - Is this indeed what will happen if the EIGRP route disappears from the primary location? And is there a way around this problem?

TOPOLOGY:

ASA-A <---eigrp---> RTR-A <-----bgp,mpls-----> RTR-B <---eigrp---> ASA-B

A and B are the two datacenters. Route is statically set on both ASA-A and ASA-B, redistributed into eigrp to neighbor routers, and then bgp picks up the routes and distributes between the datacenters.

[Julio E. Moisa]

Hi

Is required redistribute EIGRP into BGP? because if you have the route learned by EIGRP and it is into the routing table you could advertise that route via BGP without redistribution.

From my point of view modifying the AD could be the last option.  

[esa_fresa]

Hi Julio,

Sorry, I should clarify that. There's no redistribution command. The route is learned from EIGRP and advertised over BGP.

We still want to modify the AD i think, because we want the EIGRP route used before the BGP route, correct?

[Julio E. Moisa]

Hi

If you are receiving the same route via EIGRP and via BGP (iBGP AD 200, eBGP AD 20) you could modify the AD to prefer a path over other. If you are receiving the route via EIGRP only, you should not change any AD.

[esa_fresa]

Right. We're receiving the same route via eBGP, which is why we're changing the AD.

[Julio E. Moisa]

Hi,

Well if you are receiving the route via 2 routing protocols  (EIGRP, eBGP) that makes sense changing the AD, eBGP will be preferred naturally and EIGRP will be the backup, now and please correct me if Im wrong, once eBGP is down there are lost packets because the routing table has not updated by the BGP timers. In order to avoid that you could configure fast-external-fallover on the BGP neighbors  and try with BFD. 

[esa_fresa]

We're not so concerned if the BGP link goes down. We're concerned if EIGRP goes down at the primary location, that traffic will be routed in a loop between the two routers, since both routers will have the BGP routes to each other as the preferred routes.

I could be wrong on this though. My understanding is BGP does not send Update messages immediately when it's table changes, that it waits for a little bit to give any IGP's time to converge. Is the correct?

[Julio E. Moisa]

Do you have a diagram of the network with no high level?

[esa_fresa]

ASA-A <---eigrp---> RTR-A <-----bgp,mpls-----> RTR-B <---eigrp---> ASA-B

A and B are the two datacenters. Route is statically set on both ASA-A and ASA-B, redistributed into eigrp to neighbor routers, and then bgp picks up the routes and distributes between the datacenters.