This feature overrides the default protocol behavior specified in RFC-4271 related to route selection tie breakers and the definition of equal cost. In particular, routes with the same AS-path length, but differing AS numbers in the path are not considered equal cost by default. However, with multipath-relax enabled, routes with the same AS-path length can have differing AS number values in the AS-path and still be considered equal cost.
By default, BGP considers two routes as equal cost if the costs are equal through step (e) of the tie breaker procedure specified in Section 9.1.2.2 of RFC-4271. Step (a) of this procedure specifies that the AS-path length must be the same and the AS values within the path must also be the same. When the multipath-relax feature is enabled, the restriction that the AS values themselves must be the same is “relaxed”. So two routes with a different AS values in the path can be consider equal cost provided that the AS-path length is equal and the remaining criteria specified in the tie-breaker procedure [through step (e)] are the also equal. Note that multipath support must be also be enabled on the device both in BGP and RTMGR for this feature to have any effect.