BGP Timing

[Pete89]

Hello,

Last night we tested our BGP failover and it was a a success, Here is our setup:

Dual ISPs (one priamary, one for failover only), dual routers. Router 1 is connected to ISP1 and we advertise our class C out this router and recieve a default route from ISP1. On Router 2 connected to ISP2 we advertise the same class C with a longer preprend path. We also recieve a default route from ISP 2.

When we shut down Router 1 it took about 2 minutes 20 seconds to see in the looking glass that ISP 2 was advertising the Class C. When we brought back Router 1 it took less than 10 seconds for ISP 1 to start advertising and in the looking glass we could see the routes via ISP1.

Is this normal behavior? I am getting asked why initial failover to ISP2 took so long and why failback to ISP1 was so fast.

Thanks for any help explaining this.

[blau grana · Accepted solution]

Hello Pener,

It would be probably because of BGP timers. If primary router fails, ISP1 is waiting for BGP hold timer expire to flush BGP prefix. That is way it takes +-180s to reroute via backup link (ISP2).

When primary router comes UP, BGP session is established, prefix is advertised and it is immediately prefered over prefix via ISP2.

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

Best Regards

Please rate all helpful posts and close solved questions

[Pete89]

Thanks Blau Grana!

As always, you have the answers!

Tots unit fem força,

Pichi