Users Guide

Best Path Selection Criteria
Paths 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-med command 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. For example, two paths may have the same weights,
but different local preferences. BGP sees that the Weight criteria results in two potential “best paths” and moves to local
preference to reduce the options. If a number of best paths is determined, this selection criteria is applied to group’s best to
determine the ultimate best path.
In non-deterministic mode (the bgp non-deterministic-med command is applied), paths are compared in the order in
which they arrive. This method can lead to the system choosing different best paths from a set of paths, depending on the
order in which they were received from the neighbors because MED may or may not get compared between the adjacent
paths. In deterministic mode, the system compares MED between the adjacent paths within an AS group because all paths in
the AS group are from the same AS.
The following illustration shows that the decisions BGP goes through to select the best path. The list following the illustration
details the path selection criteria.
Figure 21. BGP Best Path Selection
Best Path Selection Details
1. Prefer the path with the largest WEIGHT attribute.
2. Prefer the path with the largest LOCAL_PREF attribute.
3. Prefer the path that was locally Originated via a network command, redistribute command or aggregate-
address
command.
Border Gateway Protocol IPv4 (BGPv4) 169