[c-nsp] Load-sharing with two links to the same ISP

Kevin Loch kloch at kl.net
Sat Feb 20 01:47:35 EST 2010

Matthew Melbourne wrote:
> On looking at this again, it appears that BGP Multipath only works
> when the eBGP sessions are terminated on the same box.
> The scenario here is two eBGP session to the same ISP, but terminating
> on two different customer edge routers (with an iBGP session between
> them). In the lab tests I've done, I can see the two entries in the
> BGP table (one learned via the directly connected eBGP neighbour and
> one learned through iBGP (from the other eBGP session on the other
> router), but only the best path (via the eBGP link) gets entered into
> the RIB.

That is done to prevent loops.  If you can aggregate the traffic
on other routers first, then ibgp multipath could work for you.

Another option is if the uplinks are ethernet and you are able to extend
vlans between your two routers.   Then there are several ways to
implement a full mesh (four eBGP sessions) so each of your routers would
see an equal cost path over each uplink.

- Kevin

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