BGP Convergence

[jay_7301]

Hi All,

Just had a query around BGP convergence. So from what i understand

Router 1 > Router 2 Direct EBGP session. If we shutdown the neighbor or the interface convergence should be pretty much straight away ie no waiting for timers. 

Router 1 > switch 1 > Router 2 Indirect EBGP sessions. Shutdown neighbor again will not wait for BGP timers and convergence should be quick, however if the interface went down say on R1 and the interface R3 stayed up then the timers would come into play and you would require BFD to speed up convergence. Hope i am right with the above so far?

Can anyone confirm if this is the same with IBGP and if you use VRF's would it differ from the above?

Thanks

[Reza Sharifi]

Hi,

Router 1 > Router 2 Direct EBGP session. If we shutdown the neighbor or the interface convergence should be pretty much straight away ie no waiting for timers. 

Correct. There is no waiting for timers.

Router 1 > switch 1 > Router 2 Indirect EBGP sessions. Shutdown neighbor again will not wait for BGP timers and convergence should be quick

In this case if router-1 link to switch-1 goes down, router-2 doesn't know that because the connectivity between switch-1 and router-2 is still up and so the timers have to expire before the convergence take place.  Also, it should not make any difference if you are running BGP in a VRF or global routing table.

BFD certainly speeds up the convergence time.

HTH