BGP Route 'Convergence'

I was helping cut over a new network at the weekend, topology as follows:

2 x UK sites (separate private AS No's)- each with Ethernet connection into MPLS. BGP on WAN, EIGRP on LAN. I redistribute BGP on both routers into EIGRP so that other routers inside can receive the routes to the remote LAN's. The UK sites also connect together via a primate fibre link and exchange EIGRP.

4 x European sites, same AS No's (1 with EIGRP and BGP as above due to other routers on the inside), the others with just BGP (no routers inside). The site with BGP & EIGRP uses Ethernet into MPLS, the others Leased Lines into MPLS.

On a test a colleague pulled out one of the remote WAN circuits (the one connected with Ethernet) and having plugged it back in I could not route to the remote LAN. On the main UK BGP router I had a route in my routing table and BGP table but a traceroute was looping round. First to the other UK WAN router, then up it's WAN link, then across to the WAN link of the BGP router I was on and back down to me etc.

I had to do a 'clear ip bgp' on the router and everything worked OK.

So I guess what I am asking is can anyone throw any light on why unplugging and then re-connecting (say 20 secs later) the remote WAN circuit hung the BGP table. We did wait for 5 mins to see if everything came back but to no avail. Only a clear ip bgp got the routing back on track.



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Darren Green
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