Re:[isp-bgp] How to properly utilize backup link in multi-homed setup?

["Colo Host" <ispcolohost@...>]

From: "Ejay Hire" <ejayhire@...>
> Hello.
>
> Primary and backup with bgp is easy.
>
> For inbound traffic, there are two ways to do it.
> 1.  The easy way, if you have a /23 or larger is to send your specific 
> advertisements to your primary provider, and an aggregate to the backup.  
> i.e. announce 2 /24's to your primary(ies) and the
> same netblock as a /23 to the backup provider.
Inbound is no problem, we're sending a local priority 90 community to
Level 3 on our backup link to them so that any traffic coming in from
them uses the primary link.

> Outbound control is a LOT simpler.
> Accept a default route from both providers.  use a route-map to set a 
> localpref of 90 on the backup provider's default route.
This is where my problem arises.  We don't have a "backup provider"
per se, we have two providers, Level 3 and Global Crossing, terminated
on router #1 and then we have a backup link to Level 3 terminated on
router #2, same AS.  I am sending communities on router #2 to Level 3
to cause them to lower the local preference to us for that backup link
since we're not supposed to utilize the backup link for non-bgp unless
the primary is down.  On the inside, I now have a route map on both
of my Level 3 links setting a lower metric on the primary link to them
which is working as intended, i.e. traffic that would normally go Level 3
from either router is preferring the iBGP route to router #1's Level 3
link instead of router #2 using it's own backup Level 3 link.  So that is
good.  The problem is that for routes where router #1 has chosen
Global Crossing as the best route, router #2 seems to be selecting it's
Level 3 link in some cases instead of the iBGP to router #1.  I'm not sure
why it's doing that since router #2 is configured similarly to router #1 and
should have arrived at the same conclusion as router #1 in that the path
via Global Crossing (whether over ibgp or not) is preferred.

Thanks,

D

> -Ejay Hire
>
> --- old message ---
> From: "Colo Host" <ispcolohost@...>
> Date: Sun, 05 Dec 2004 13:09:38 -0500
> X-Message-Number: 1
>
> Hello all, I'm hoping someone could give me a suggestion on how
> to only make use of a backup link when the primary is down.  The
> setup is two border routers, router #1 and router #2.  #1 has
> connectivity to Level 3 (ASN 3356) and Global Crossing (ASN 3549).
> We've just added router #2 and it has a link to Level 3 (ASN 3356)
> which is to only be used if the primary link to them were to fail and
> will soon have a third provider brought on.
>
> --- snip ---
>
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