BGP - Multipath Attribute Prerequisites
When implementing multipath load sharing as the load balancing method for BGP, certain BGP attributes must be the same for the candidate routes to be considered for multipath.
However, this list of prerequisites is not defined in any official RFC. It is highly vendor-specific and implementation-specific. Although there is a general consensus among most vendors, especially for the first five prerequisites, there seems to be some discrepancy about the use of the IGP metric as part of the prerequisites for multipath load sharing in BGP.
As far as Cisco goes, the BGP configuration guide (see link below) states that the IGP distance to the next hop must be the same for all routes to be candidates for multipath routing. Other documentation that examines the BGP best path algorithm seems to indicate that the IGP metric is not always used, and may be implementation-specific.
In practice, we see that IGP distance to the next hop is not always a prerequisite. Thus, each scenario must be thoroughly tested in order to determine the actual behavior in a real-world scenario before implementing on a production network.
Links:
https://forum.networklessons.com/t/bgp-multipath-load-sharing-ibgp-and-ebgp/3275/52?u=lagapidis