Aggressive BGP Fall-Over Behavior

Soon after I wrote the Designing Fast Converging BGP Networks article (you’ll find it somewhere in this list, one of my regular readers sent me an interesting problem: BGP sessions would be lost in his (IS-IS based) core network if he would use fall-over on IBGP neighbors and the BGP router would have a primary and a backup path to the IBGP neighbor.

It turned out to be an interesting side effect of aggressive route table purge following a link failure: the route to BGP neighbor was removed from the routing table before IS-IS ran SPF and installed an alternate route, and BGP decided it’s time to give up and terminate the session.

For more details, read the What Exactly Happens after a Link Failure blog post.


  1. I can't find some documentations on fall-over bfd...
    Can someone please explain "neighbor x.x.x.x fall-over bfd" ???


    Lewy Dalumpines
    1. How about following the links in my blog post? In particular, this one might answer your question ;)
Add comment