Reference Guide

• Origin
• AS Path
• Next Hop
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 FTOS 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, FTOS compares MED between the adjacent paths within an AS group because all
paths in the AS group are from the same AS.
NOTE: In FTOS version 8.3.11.4, the bgp bestpath as-path multipath-relax command is disabled by
default, preventing BGP from load-balancing a learned route across two or more eBGP peers. To enable load-
balancing across different eBGP peers, enable the bgp bestpath as-path multipath-relax
command. A system error results if you configure the bgp bestpath as-path ignore command and the
bgp bestpath as-path multipath-relax command at the same time. Only enable one command at a
time.
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.
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