How can I multipath BGP prefixes of equal AS-path length?

Question

Say I have two routes of 10.0.0.0/8. One has an AS-path of 100 200 300, the other has 400 500 300. How can I make BGP install both routes into the FIB on a Cisco router?

Answer 1

Cisco has a command that will do this although it's technically undocumented.

Within the the context of router bgp <your AS> you should enter bgp best-path as-path multipath-relax

This will cause it to multipath routes that satisfy the usual requirements (i.e. equal weight, MED and LocPref) but only requires the AS-path to be the same length rather than identical.

On a side note, there is also bgp best-path as-path ignore - unfortunately this does not permit multipathing regardless of AS path and will instead entirely prevent it.