BGP fast Convergence time

Unanswered Question
Jul 23rd, 2009

Hi Friends,

i have dual router multihomed network from 2 different ISP and advertising just one /24 network in BGP that is acceptable from both ISPs.

now i need fast convergence in BGP because if ISP-1 link goes down it takes approx 1 min. to route the traffic from ISP-2.

i have got one document stating about FALLOVER feature to increase the BGP convergence time.

but i m not sure about the impact of this command.

is it right to configure this in running network.

plz help me.



I have this problem too.
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guruprasadr Thu, 07/23/2009 - 03:13

HI Nitin,

With this feature, you are bypassing the standard BGP scanning intervals.

It is not recommended in the large-scale Networks.

Hope this Helps.

Best Regards,

Guru Prasad R.

nitsg.gupta Thu, 07/23/2009 - 03:19

thanks for the reply.

i know that manipulating the BGP convergence it may increase TCP traffic and consequently increase CPU load but i m not geeting specific routes from ISP, i m just getting 1 default route from both the ISPs.

Laurent Aubert Fri, 07/24/2009 - 06:45

Hi Nitin,

This command is useful if you are learning your BGP peer address via an IGP which is not the case here. I assume your eBGP session are built on the interface IP addresses.

I don't think you have a convergence issue on your side because as soon as the link failed, the router will install its backup route. The 1mn is the time for the internet to converge for the returning traffic and I don't think there is something you can do.



John Blakley Fri, 07/24/2009 - 07:06


If you're using eBGP, the convergence times don't work like normal holddown timers for an IGP. Once your provider sees the link in a down/down state, it will automatically failover to the other router. It will still keep trying to contact the primary router until it completely times out, but your backup router will be used. You don't have to mess with timers to get this to work.

I ran into this problem with our ISP because I didn't want it to failover so quick, but I soon learned that it wasn't something I could control.




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