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:
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.
Border Gateway Protocol IPv4 (BGPv4)
171