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

["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.

2.  The not-so-easy way, announce the routes to all providers, and set a 
community on the route announced to the backup provider that causes them to 
set a localpref lower than that of the routes they learn through their other 
peers.

i.e. you send a route to mci with the 701:90  community.  They set a 
localpref of 90 on that route inside their network.  they learn about the 
same network through their peer connections, with a default localpref of 
100.  they will use the peer route instead of your connected route.

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.

-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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