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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:
a. This comparison is only done if the first (neighboring) AS is the same in the two paths; the MEDs
are compared only if the first AS in the AS_SEQUENCE is the same for both paths.
b. If you entered the bgp always-compare-med command, MEDs are compared for all paths.
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Border Gateway Protocol IPv4 (BGPv4)