route converge time

Baldur Norddahl baldur.norddahl at
Sat Nov 21 13:44:35 UTC 2015


I got a network with two routers and two IP transit providers, each with
the full BGP table. Router A is connected to provider A and router B to
provider B. We use MPLS with a L3VPN with a VRF called "internet".
Everything happens inside that VRF.

Now if I interrupt one of the IP transit circuits, the routers will take
several minutes to remove the now bad routes and move everything to the
remaining transit provider. This is very noticeable to the customers. I am
looking into ways to improve that.

I added a default static route to provider A on router A and did
the same to provider B on router B. This is supposed to be a trick that
allows the network to move packets before everything is fully converged.
Traffic might not leave the most optimal link, but it will be delivered.

Say I take down the provider A link on router A. As I understand it, the
hardware will notice this right away and stop using the routes to provider
A. Router A might know about the default route on router B and send the
traffic to router B. However this is not much help, because on router B
there is no link that is down, so the hardware is unaware until the BGP
process is done updating the hardware tables. Which apparently can take
several minutes.

My routers also have multipath support, but I am unsure if that is going to
be of any help.

Anyone got any tricks or pointers to what can be done to optimize the
downtime in case of a IP transit link failure? Or the related case of one
my routers going down or the link between them going down (the traffic
would go a non-direct way instead if the direct link is down).



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