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 Dell Networking OS 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,
Dell Networking OS compares MED between the adjacent paths within an AS group because all paths in the AS group are from the same
AS.
NOTE: 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.
Figure 20. 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.
a Routes originated with the Originated via a network or redistribute commands are preferred over routes
originated with the
aggregate-address command.
4 Prefer the path with the shortest AS_PATH (unless the bgp bestpath as-path ignore command is configured, then
AS_PATH is not considered). The following criteria apply:
Border Gateway Protocol IPv4 (BGPv4)
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