Re: BGP and convergence time

[Danny McPherson <danny () tcb net>]

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