Border Gateway Protocol IPv4 (BGPv4) | 211BGP AttributesRoutes learned via BGP have associated properties that are used to determine the best route to a destinationwhen multiple paths exist to a particular destination. These properties are referred to as BGP attributes, andan understanding of how BGP attributes influence route selection is required for the design of robustnetworks. This section describes the attributes that BGP uses in the route selection process:• Weight• Local Preference• Multi-Exit Discriminators (MEDs)• Origin• AS Path• Next HopBest Path Selection CriteriaPaths for active routes are grouped in ascending order according to their neighboring external AS number(BGP best path selection is deterministic by default, which means the bgp non-deterministic-medcommand is NOT applied).The best path in each group is selected based on specific criteria. Only one “best path” is selected at a time.If any of the criteria results in more than one path, BGP moves on to the next option in the list. Forexample, two paths may have the same weights, but different local preferences. BGP sees that the Weightcriteria results in two potential “best paths” and moves to local preference to reduce the options. If anumber 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.Figure 10-4 illustrates the decisions BGP goes through to select the best path. The list following theillustration details the path selection criteria.
