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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:
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 first 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 (first received path). For paths containing a route reflector (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 configuration 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 Networking OS choosing different 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.
Border Gateway Protocol IPv4 (BGPv4)
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Troubleshooting
