182 | Border Gateway Protocolw w w . d e l l . c o m | s u p p o r t . d e l l . c o m 11. Prefer the external path originated from the BGP router with the lowest router ID. If both paths areexternal, prefer the oldest path (first received path). For paths containing a Route Reflector (RR)attribute, the originator ID is substituted for the router ID.12. If two paths have the same router ID, prefer the path with the lowest cluster ID length. Paths without acluster ID length are set to a 0 cluster ID length.13. Prefer the path originated from the neighbor with the lowest address. (The neighbor address is used inthe BGP neighbor configuration, and corresponds to the remote peer used in the TCP connection withthe local router.)After a number of best paths is determined, this selection criteria is applied to group’s best to determine theultimate best path.In non-deterministic mode (the bgp non-deterministic-med command is applied), paths are compared inthe order in which they arrive. This method can lead to FTOS choosing different best paths from a set ofpaths, depending on the order in which they were received from the neighbors since MED may or may notget compared between adjacent paths. In deterministic mode, FTOS compares MED between adjacentpaths within an AS group since all paths in the AS group are from the same AS.WeightThe Weight attribute is local to the router and is not advertised to neighboring routers. If the router learnsabout more than one route to the same destination, the route with the highest weight will be preferred.Theroute with the highest weight is installed in the IP routing table.Local PreferenceLocal Preference (LOCAL_PREF) represents the degree of preference within the entireAS. The higher thenumber, the greater the preference for the route.The Local Preference (LOCAL_PREF) is one of the criteria used to determine the best path, so keep inmind that other criteria may impact selection, as shown in Figure 9-4. For this example, assume thatLOCAL_PREF is the only attribute applied. In Figure 9-5, AS100 has two possible paths to AS 200.Although the path through the Router A is shorter (one hop instead of two) the LOCAL_PREF settingshave the preferred path go through Router B and AS300. This is advertised to all routers within AS100causing all BGP speakers to prefer the path through Router B.
