[c-nsp] BGP neighbor fall-over vs BFD

[Steven Raymond]

On Mar 11, 2013, at 10:52 AM, John Neiberger wrote:

> I was just reading a bit about next-hop tracking and neighbor fall-over and
> now I'm a little confused about what fall-over actually does. The docs say
> that it enables fast peering session deactivation, but I can't tell what
> that really means. The wording in the docs makes it sound a lot like BFD,
> but not exactly. In fact, fall-over can be used with BFD.
> 
> Can someone shed some light on this? What is fall-over really doing and
> when might it be useful?

BFD is useful for detecting failure of links between directly-connected neighbors.  

Fall-over is useful for immediate notification of distant IBGP neighbors.

So while BFD will inform your immediate neighbor of link down, the rest of the IBGP has to then wait for your BGP timeout to drop.  Yes, that is slow.  FYI, Brocade MLX-series does not have this feature, so in a mixed environment I ended up tweaking the keepalive timers on the Brocade neighbors to compensate.