[c-nsp] Link down affecting BGP peer

[Saku Ytti]

On Thu, 19 May 2022 at 10:27, Hank Nussbacher <hank at interall.co.il> wrote:

> Others have explained this.  Basically, a BGP peer gets locked onto one
> of the LAG links and will migrate to another link in the event that the
> specific link goes down.  This is normal behavior.

I'm not sure about normal behaviour and certainly objectively broken.

Even though ultimately some physical interfaces serialise those BGP
packets out, the fast external fallover should be tracking the
aggregate interface, not some member. What should happen when a member
comes down is that the hash=>interface table has one interface less,
so the packet is now hashed out to some of the remaining interfaces.

We can accept flaps if we don't know the physical interface is down,
while it is down. Like if the carrier-delay down is higher than bgp
keepalive or if the interface is blackholing for whatever reason.
Other than that, no, it's broken.

-- 
  ++ytti