BGP Multipath load-sharing in Cisco!

Some years ago I was consulting a project and there the team faced an issue with load-sharing the outbound traffic towards different eBGP neighbors from different ASs. They reached out to me and it took me some times to find the solution. Suddenly I was skimming through my documents and saw that, so I’m gonna share it here too.

When implementing BGP in a Cisco environment, you may want to load-share the outgoing traffic between multiple next-hops which you have. The first command which probably you are thinking of is maximum-path 4  to use 4 different paths.

Yeah, that’s somehow true, but it requires the following attributes to match:

  • weight
  • local preference
  • AS path
  • origin code
  • MED,
  • IGP metric.

And for sure, the next-hop address for each path must also be different in order for that path to be considered. (imagine when multi-homing to the same router)

The point here is that, the router should receive the routes from the same AS.

So, what if we wanna load-share between different eBGP neighbors from different ASs?

Now we are delighted with an undocumented (unsupported) Cisco command:

csr1000v(config-router)#bgp bestpath as-path multipath-relax
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Author: Mo Moghaddas

Traveler. Casual investor. Hobbyist landscape photographer loving Networks. Blockchain/Cryptocurrency enthusiast. Internet citizen!

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