Users Guide

Figure 21. 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.
9. Dell Networking OS deems the paths as equal and does not perform steps 9 through 11, if the following criteria is met:
a. the IBGP multipath or EBGP multipath are configured (the maximum-path command).
b. the paths being compared were received from the same AS with the same number of ASs in the AS Path but with different
NextHops.
c. the paths were received from IBGP or EBGP neighbor respectively.
10. If the bgp bestpath router-id ignore command is enabled and:
Border Gateway Protocol IPv4 (BGPv4)
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