[126338] in North American Network Operators' Group
Re: BGP and convergence time
daemon@ATHENA.MIT.EDU (Danny McPherson)
Wed May 12 11:53:01 2010
From: Danny McPherson <danny@tcb.net>
In-Reply-To: <AANLkTikeWFppYU68GRcJVxihFhZwibCaekmfCG-UWv2l@mail.gmail.com>
Date: Wed, 12 May 2010 09:52:48 -0600
To: North American Network Operators Group <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On May 12, 2010, at 9:40 AM, Jay Nakamura wrote:
>
> I just tested this and, yes, with Cisco to Cisco, changing the setting
> won't reset the connection but you have to reset the connection to
> have the value take effect. I need to look up what happens when two
> sides are set to different values and which one takes precedent.
The holdtime isn't technically negotiated, both sides convey their
value in the open message and the lower of the two is used by both
BGP speakers. IIRC, neither J or C reset the session with the timer
change, but the new holdtimer expiry value doesn't take effect until
then.
One other thing to note is that by default, keepalive intervals in
those implementations are {holdtime/3}. Normally, if you're setting
holdtime to something really lower (e.g., 10 seconds) you might want
to increase the frequency of keepalives such that the probability of
getting one through in times of instability rise. In particular,
congestion incurred outside of BGP, as update messages themselves
will serve as implicit keepalives, and with the amount of churn in BGP,
empty updates (keepalives) are rare for most speakers with a global BGP
view.
-danny
