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order in which they were received from the neighbors because MED may or may not get compared between the adjacent paths.
In deterministic mode, the system compares MED between the adjacent paths within an AS group because all paths in the AS
group are from the same AS.
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:
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.
c. Paths with no MED are treated as “worst” and assigned a MED of 4294967295.
7. Prefer external (EBGP) to internal (IBGP) paths or confederation EBGP paths.
8. Prefer the path with the lowest IGP metric to the BGP if next-hop is selected when synchronization is disabled and
only an internal path remains.
Border Gateway Protocol IPv4 (BGPv4)
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