Glossary

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 EMC 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 EMC 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 19. 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:
a. An AS_SET has a path length of 1, no matter how many ASs are in the set.
b. A path with no AS_PATH configured has a path length of 0.
c. AS_CONFED_SET is not included in the AS_PATH length.
d. AS_CONFED_SEQUENCE has a path length of 1, no matter how many ASs are in the AS_CONFED_SEQUENCE.
5. Prefer the path with the lowest ORIGIN type (IGP is lower than EGP, and EGP is lower than INCOMPLETE).
6. Prefer the path with the lowest multi-exit discriminator (MED) attribute. The following criteria apply:
Border Gateway Protocol (BGP)
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