route converge time

Sat Nov 28 19:34:35 UTC 2015

One thing I notice you don't mention is whether your
BGP sessions to your upstream providers are direct
or multi-hop eBGP.  I know for a while some of the
more bargain-basement providers were doing eBGP
multi-hop feeds for full tables, which will definitely
slow down convergence if the routers have to wait
for hold timers to expire to flush routes, rather than
being able to direct detect link state transitions.

Matt

On Sat, Nov 21, 2015 at 5:44 AM, Baldur Norddahl
<baldur.norddahl at gmail.com> wrote:
> Hi
>
> 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 0.0.0.0 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).
>
> Thanks,
>
> Baldur
>