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

[Pete Templin <petelists@...>]

Colo Host wrote:
> 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.
>
> I don't think setting local preference lower on the backup link will
> work because I believe local preference is not AS-specific and
> would cause all outbound traffic to prefer Global Crossing on router
> #1 if the primary Level 3 link were to fail since it would have a
> higher local preference than the backup Level 3 link.  I'm thinking
> I could get Level 3 to send me MEDs to prefer the primary link
> but is there any other way I can force it myself on my side?
MED is your friend.  Unlike weight and local pref, low MED wins.  Set a
low MED (say 50) on all announcements to L3 over the primary link and on
all routes learned from L3 over the primary link, and a high MED (say
1,000,000) on all announcements to L3 over the backup link and on all
routes learned from L3 learned from L3 over the backup link.

If they do any MED twiddling in their network, the large difference in
values should suffice.  You may see trace amounts of traffic over the
backup link, for BGP updates and any traffic due to slightly
unsynchronized BGP tables, etc.

Remember that outbound MED must be set on the EBGP egress point, or
it'll get reset to 0 at the EBGP egress point.  Inbound MED can be set
anywhere, but remember that it'll get flushed if/when it leaves your
network again.

Ought to work like a charm...

pt







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