BGP keepalive / holdtime and best path selection

[jeffreywatson]

If I have a router that peers with two eBGP neighbors RTRA and RTRB.

The keepalive and holdtime is set to 30/90.

The router has two valid routes to a destination network

The one via the RTRB Peer is VALID and BEST.

The one via the RTRA Peer is VALID

if RTRB goes down how long should I expect it to take for the next Valid route via RTR A be recognized as Valid and BEST and  be put into the RIB ?   

90 seconds + ??? or should that happen very fast ??

is there some other timer involved

thanks in advance

[milan.kulik]

Hi,

generally, if your router interface is still Up, it might really take over 90 seconds.

You could improve the dead neighbor detection by decreasing the timers or by configuring BFD on the routers, see

http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_bfd.html#wp1106001

Best regards,

Milan

[jeffreywatson]

Thanks - I have to see if the SP we interface to will support that.

We have some Date processors that bind to each other through either one of these links depending on the routing.  And their timeout is set at 90 seconds

if I set the BGP timers to 20 & 60 seconds then I should recognize the prime path via RTR B down within 60 seconds.  Will the BGP process then proceed to put the next VALID path into the RIB within a few seconds or is there some other timer that has to expire or update packet that has to be received via RTRA?

router bgp xxxx

bgp timers 20 60

[Richard Burts · Accepted solution]

Once BGP has determined that its current "best" route is no longer valid it should converge to the alternate route pretty quickly.

HTH

Rick

[jeffreywatson]

ok thank you very much -

I have to try it out I guess

I'm assuming that as soon as RTRB is down and the holdtime timer expires then our router will recognize that the neighbor is down. 

And at that point all the BGP routes that are currently Valid and Best Via RTRB will be recognized as no longer "Valid".   

Then RTA will put the next the next Valid route in immediately

[milan.kulik · Accepted solution]

Hi,

here is a nice document, look at the BGP Next-Hop Tracking in the end of it.

http://networkgeekstuff.com/networking/cisco-bgp-timers-re-explained/

Best regards,

Milan

[jeffreywatson]

awesome thanks !!