eBGP Multihop

Steven King sking at kingrst.com
Thu Sep 2 14:50:46 UTC 2010

 The last company I worked for moved to eBGP Multi-Hop where there were
two connections to the same provider (same AS). This allowed them to
utilize both links in both directions vs only one link in one direction
and have failover.

As you have mentioned link state detection gets a bit crazy with this.
If you have a MetroE connection (for example) with multiple segments,
this could be problematic. If your side of the link goes down, then you
stop sending traffic to the provider, but the provider still tries to
send traffic to you. If a segment in the middle goes down, then neither
side stop sending traffic.

Due to the fact that the BGP session is still up, and the interface on
your router is still up, BGP sees the link as a valid path.

However there is a fix for this. If your provider supports it that is.
Ethernet OAM (Ethernet Operations, Administration, and Management) will
allow you to monitor the connection on Layer 2 end to end and not switch
to switch. If any part of the link breaks, OAM brings your and the other
side of the link down, telling BGP that the link is no longer usable,
therefore avoiding the issues above.

If you are using a POS, MPLS, or other similar technology, then the
issues talked about above are either less of an issue, or not an issue
at all. The biggest problem with multi segment Ethernet links is that
you need OAM to reliably run eBGP Multihop and OAM isn't supported by a
lot of providers (mainly because it requires a newer software version).

Hope this helps.

On 9/2/10 5:30 AM, Graham Beneke wrote:
> I have been asked to investigate moving an entire network to multi-hop
> on all the eBGP sessions. Basically all upstreams, downstreams and
> peers will eBGP with a route reflector located in the core. This RR
> will be some kind of quagga or similar box. The dev guys want to be
> able to poke at the BGP feeds directly and do *magic* that standard
> router aren't capable of.
> My gut feel is that this is a bad idea. Besides anything else it makes
> sane link state detection very challenging - especially where we have
> multiple sessions with a peer.
> Is their any BCP or operational experience that agrees or disagrees
> with my gut. ;-)

Steve King

Senior Linux Engineer - Advance Internet, Inc.
Cisco Certified Network Associate
CompTIA Linux+ Certified Professional
CompTIA A+ Certified Professional

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