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 Dell Networking OS choosing dierent 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, Dell
Networking OS compares MED between the adjacent paths within an AS group because all paths in the AS group are from the same AS.
NOTE: 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 dierent eBGP peers, enable the bgp
bestpath as-path multipath-relax
command. A system error results if you congure 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 congured, 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 congured 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.
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.
172
Border Gateway Protocol IPv4 (BGPv4)
To Next Page
Loading...
Need help?
General
