For each prefix in the routing table, the routing protocol selects the best path. Then, the best path is compared to the next path in the list until all paths in the list are exhausted. The following parameters are used to determine the best path:
Routes are not considered if they are unreachable.
An RTM’s preference is lowered as well as the hierarchy of routes from a different protocol. The lower the preference the higher the chance of the route being the active route.
Routes with higher local preference have preference.
Routes with the shorter AS path have preference.
Routes with the lower origin have preference. IGP = 0 EGP = 1 INCOMPLETE = 2
Routes with the lowest MED metric have preference. Routes with no MED value are exempted from this step unless always-compare-med is configured.
Routes learned by an eBGP peer rather than those learned from an iBGP peer are preferred.
Routes with the lowest IGP cost to the next-hop path attribute are preferred.
Routes with the lowest BGP-ID are preferred.
Routes with shortest cluster list are preferred.
Routes with lowest next-hop IP address are preferred.
For BGP-VPN routes with the same prefix but a different Route Distinguisher (RD) that are imported in a VRF, if ECMP is not enabled in that VRF, the preceding selection criteria are used until parameter point 8. If all selection criteria are still the same after that point, the last updated route will be selected.
For BGP-VPN routes with the same prefix but a different Route Distinguisher (RD) that reach parameter point 8 in the selection criteria, all routes are flagged as BEST and USED, although the actual number of used routes depends on the ECMP value configured in the VRF.
7210 SAS devices do not support BGP ECMP (multi-path). That is, an ECMP value of 1 is always used.
For BGP-VPN routes with the same prefix and same Route Distinguisher (RD) that reach parameter point 8 in the selection criteria, such routes are flagged as BEST but parameter points 9 to 11 determine which routes are submitted to the VRF and marked as USED in accordance with the ECMP value configured in the VRF.