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.
Bzzzt...I would recommend against this. If your primary L3 link goes
down, the routes you advertise to GBLX will be preferred inside L3 MORE
than your routes from your backup link. Send high MED on the backup L3
link and low MED on the primary L3 link, and do the same on what you
learn from L3.
Otherwise, failure of your primary L3 link will likely dump all of your
inbound traffic to GBLX, leaving your backup L3 link for ONLY those
times when both L3(pri) and GBLX are "down" (or R1 is down, etc.).
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.
R2 is probably choosing L3(backup) over GBLX by the "prefer the external
BGP path over the internal BGP path" step in the path selection
algorithm. The best way to fix this may depend on your internal
topology (i.e. you may want/need to shift R2 to an edge-only role, so
that no local traffic traverses it and then wants to leave via L3). How
is your internal net set up?
pt
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Re: [isp-bgp] How to properly utilize backup link in multi-homed setup?, Colo Host
