Concept Guide

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.
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 EMC 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 congured (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 dierent
NextHops.
c the paths were received from IBGP or EBGP neighbor respectively.
10 If the bgp bestpath router-id ignore command is enabled and:
a if the Router-ID is the same for multiple paths (because the routes were received from the same route) skip this step.
b if the Router-ID is NOT the same for multiple paths, prefer the path that was rst received as the Best Path. The path selection
algorithm returns without performing any of the checks detailed here.
11 Prefer the external path originated from the BGP router with the lowest router ID. If both paths are external, prefer the oldest path
(rst received path). For paths containing a route reector (RR) attribute, the originator ID is substituted for the router ID.
12 If two paths have the same router ID, prefer the path with the lowest cluster ID length. Paths without a cluster ID length are set to a 0
cluster ID length.
13 Prefer the path originated from the neighbor with the lowest address. (The neighbor address is used in the BGP neighbor
conguration and corresponds to the remote peer used in the TCP connection with the local router.)
After a number of best paths is 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 Dell EMC Networking OS 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, Dell EMC Networking OS compares MED between the adjacent paths within an AS group because all paths in the AS group are
from the same AS.
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Border Gateway Protocol (BGP)