Administrator Guide

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 are 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 the system choosing dierent 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, the system
compares MED between the adjacent paths within an AS group because all paths in the AS group are from the same AS.
NOTE: In the Dell Networking OS version 8.3.11.4, 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
dierent eBGP peers, enable the bgp bestpath as-path multipath-relax command. A system error results if you
congure 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 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 congured, 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 congured 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 rst (neighboring) AS is the same in the two paths; the MEDs are compared only if the rst
AS in the AS_SEQUENCE is the same for both paths.
Border Gateway Protocol IPv4 (BGPv4)
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