[186119] in North American Network Operators' Group
Re: route converge time
daemon@ATHENA.MIT.EDU (Jeff Tantsura)
Sat Nov 28 16:59:54 2015
X-Original-To: nanog@nanog.org
From: Jeff Tantsura <jeff.tantsura@ericsson.com>
To: Matthew Petach <mpetach@netflight.com>
Date: Sat, 28 Nov 2015 21:59:49 +0000
In-Reply-To: <CAEmG1=ri=-Bnay4GbbCxKRdzRAAfkQYzUArVXqQ_NQ3ab+RDuQ@mail.gmail.com>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
In that case multihop BFD (if supported on both sides) would really help.
Regards,
Jeff
> On Nov 28, 2015, at 11:37 AM, Matthew Petach <mpetach@netflight.com> wrot=
e:
>=20
> 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.
>=20
> Matt
>=20
>=20
> On Sat, Nov 21, 2015 at 5:44 AM, Baldur Norddahl
> <baldur.norddahl@gmail.com> wrote:
>> Hi
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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 provid=
er
>> 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.
>>=20
>> My routers also have multipath support, but I am unsure if that is going=
to
>> be of any help.
>>=20
>> 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 on=
e
>> 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).
>>=20
>> Thanks,
>>=20
>> Baldur
>>=20
