Re: BGP Keepalive Timers

It's because the BGP process and its keepalives are software switched on the CPU. If the CPU gets busy with something like BGP convergence then the session will flap. BFD offloads the processing in hardware to the linecard, so you get fast linkdown detection without the danger of the CPU accidentally dropping the session.

This is true of all protocols, e.g. OSPF, EIGRP, IS-IS, etc. It's not a BGP problem it's a process switching problem.

Brian McGahan, 4 x CCIE #8593 (R&S/SP/SC/DC), CCDE #2013::13
bmcgahan_at_INE.com<mailto:bmcgahan_at_INE.com>

Internetwork Expert, Inc.
http://www.INE.com<http://www.ine.com/>

On Apr 17, 2014, at 4:50 AM, "Dave Serra" <maybeedave_at_yahoo.com<mailto:maybeedave_at_yahoo.com>> wrote:

Hi Guys,

We have an extranet with about 20 or so partners and we peer BGP
with them. Combined we only have a few hundred routes in the BGP table from
them. While in conversation with one of them for new connectivity I wanted to
use keepalive timers of 1 and 3 and the provider said this is way too small
and will produce instability in BGP meaning BGP may flap. Sure enough when I
tried to configure it I also got a warning that a hold down timer of less then
20 seconds would cause BGP to flap. My question is why? I keep hearing the
answer that the circuits could flap and trigger failover but failover is
desirable as the application has a 30 second timeout before it resets the TCP
connection. I told this to the provider and he was talking about the BGP
process not being able to process all of the keepalives but using BFD we could
do this. I don't get why the BGP process would have an issue with 20 or 40
BGP sessions all sending 1 keepalive per second.
This is such a small amount of traffic. Someone at work also mentioned there
may be policing on the cat6500 that keeps all of the hellos from getting to
the CPU and too many sessions can cause ANY of your BGP sessions to flap but I
that also doesn't sound right.

Anyway, if anyone knows why the BGP process
wouldn't be able to handle 1 and 3 keepalives for say 20 or even 40 peers I
would be very interested to hear more.

Thanks again!

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Received on Thu Apr 17 2014 - 08:19:36 ART