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

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

From: "Ejay Hire" <ejayhire@...>
> 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.
Almost. :-)  I don't have a backup provider, I have a backup link to one of
the two providers and only want to utilize the primary link except when it's
down but without affecting normal outbound traffic decisions.  So I can't
change local preference because then if the primary link to provider #1 went
down, all of my traffic would shift over to just provider #2 anyway since it
would still have a lower local preference than the backup link to provider 
#1.

What I've done to get around this is to forcibly override the incoming
MEDs from provider #1 to set them lower on primary link than on the
backup link and left provider #2's MEDs alone since they're not considered
unless you have always compare MEDs turned on to make it non-deterministic.
The only problem I seem to have left though is that router #1 seems to be
choosing Global Crossing in some cases while router #2 is chosing its
backup-only link to Level 3 instead of the iBGP route to router #1 and
Global Crossing.  I'm not sure why this is happening because the AS path
lengths for the iBGP route and the local Level 3 route are the same, and
without nondeterministic MED comparison set, BGP should always choose
the IGP over the EGP in such a case.  So I'm still working on this... :-)

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 ---
>
> _________________________________________________________________
> Don’t just search. Find. Check out the new MSN Search! 
> http://search.msn.click-url.com/go/onm00200636ave/direct/01/



> To unsubscribe via postal mail, please contact us at:
> Jupitermedia Corp.
> Attn: Discussion List Management
> 475 Park Avenue South
> New York, NY 10016
>
> Please include the email address which you have been contacted with.







To unsubscribe via postal mail, please contact us at:
Jupitermedia Corp.
Attn: Discussion List Management
475 Park Avenue South
New York, NY 10016

Please include the email address which you have been contacted with.