[c-nsp] ebgp load balancing using maxiumu-paths TCAM impact on Sup720-3BXL?

Kevin Hodle kevin.hodle at gmail.com
Thu May 21 12:15:44 EDT 2009

This is correct - the primary benefit of multi-hop in most topologies
would be on a downstream iBGP speaker to (for example, a core facing
route-reflector) with equal IGP costs to each edge router - this would
achieve proper outbound load balancing. Also worth noting is that this
will only be useful if Peter continues to use a single provider for
his multiple sessions. If, one day Peter decides to bring in a second
carrier for redundancy purposes multi-path will no longer load balance
since as-path's will now be different. There is a hidden IOS command
in newer releases to get around this: bgp bestpath as-path
multipath-relax that will permit multi-path table installation of
identical routes ignoring the as-paths attribute. As you continue to
add diverse upstreams in multiple locations however, this will not
scale too well. Eventually you will need to start doing some traffic
engineering via as-path + local-pref or other method to nail-down
upstream route preferencing if you want to meet commits on all these
diverse ports.

Kevin Hodle

On Thu, May 21, 2009 at 9:27 AM, Pete Templin <petelists at templin.org> wrote:
> Phil Mayers wrote:
>> I'm not 100% certain about this, but my understanding is that there is
>> still only 1 FIB entry for the route; it just has >1 next hop.
>> However - will this work? For eBGP-multipath, the paths have to be
>> basically identical except next-hop. This won't be the case, since each
>> router will prefer its direct link (lower IGP cost).
> I think you're close: rule #7 says "Prefer eBGP over iBGP paths", so each
> router will prefer its direct link (since it's external).  The indirect
> paths won't be available for parallel path selection since they've fallen
> out before the bottom.
> pt
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