Re: BGP AFI or SAFI for advertising BFD status

[Nick Hilliard <nick () foobar org>]

Tom Beecher wrote on 22/12/2024 02:51:
> Definitely will be interesting to read the list discussion about this. 
> My first reaction was why would you even need this, so def curious.

There are two main target situations: firstly, when a router 
unexpectedly drops off an ixp platform, this won't be explicitly 
signaled to the other routers on the fabric, which can mean that packets 
to that device will be black-holed until all the others bgp hold timers 
kick in. This would typically happen after 90-180s (e.g. hello time: 
30-60s). The second situation would be to deal with forwarding plane 
incongruence on IXPs, i.e. where router A can reach RS, router B can 
reach RS, router A cannot reach router B due to a problem on the IXP 
fabric itself. Thankfully this style of problem has become quite unusual 
over the last several years.

I'm not sure it's a all-round good solution to either of these problems, 
in the "be careful what you wish for, because you might get it" sense. 
There are going to be router platforms out there which won't handle 
hundreds of BFD sessions reliably, so if the protocol were widely 
supported, it's not clear that it would help or harm interdomain routing 
stability due to the ability of routers to handle large numbers of BFD 
sessions, particularly where there were situations where all the 
sessions could be triggered simultaneously.

As a separate issue, hold timers should generally be of a comparable 
order of magnitude to the non-availability effect they're attempting to 
mitigate. Inter-domain routing convergence is often measured in minutes 
rather than seconds. So even if the protocol layer worked at IXPs 
without causing control plane meltdown, it's still a mechanism which 
which has a trigger timer two orders of magnitude faster than the 
general case of DFZ reconvergence. I can't see that this would help 
overall inter-domain routing stability.

Nick