Cisco Nexus 3000 Series Configuration Manual page 137

Chapter 5 Configuring Basic BGP
S e n d d o c u m e n t c o m m e n t s t o n e x u s 3 k - d o c f e e d b a c k @ c i s c o . c o m
Cisco NX-OS chooses the path with the highest weight.
2.
Cisco NX-OS chooses the path with the highest local preference.
3.
If one of the paths is locally originated, Cisco NX-OS chooses that path.
4.
Cisco NX-OS chooses the path with the shorter AS path.
5.
Note
Cisco NX-OS chooses the path with the lower origin. Interior Gateway Protocol (IGP) is considered
6.
lower than EGP.
Cisco NX-OS chooses the path with the lower multi- exit discriminator (MED).
7.
You can configure a number of options that affect whether or not this step is performed. In general,
Cisco NX-OS compares the MED of both paths if the paths were received from peers in the same
autonomous system; otherwise, Cisco NX-OS skips the MED comparison.
You can configure Cisco NX-OS to always perform the best-path algorithm MED comparison,
regardless of the peer autonomous system in the paths. See the
section on page 6-8
comparison that depends on the AS-path attributes of the two paths being compared:
a.
b.
c.
d.
Note
e.
If one path is from an internal peer and the other path is from an external peer, then Cisco NX-OS
8.
chooses the path from the external peer.
If the paths have different IGP metrics to their next-hop addresses, then Cisco NX-OS chooses the
9.
path with the lower IGP metric.
Cisco NX-OS uses the path that was selected by the best-path algorithm the last time that it was run.
10.
When calculating the length of the AS path, Cisco NX-OS ignores confederation segments, and
counts AS sets as 1. See the
for more information. Otherwise, Cisco NX-OS will perform a MED
If a path has no AS path or the AS path starts with an AS_SET, then the path is internal, and
Cisco NX-OS compares the MED to other internal paths.
If the AS path starts with an AS_SEQUENCE, then the peer autonomous system is the first AS
number in the sequence, and Cisco NX-OS compares the MED to other paths that have the same
peer autonomous system.
If the AS path contains only confederation segments or starts with confederation segments
followed by an AS_SET, the path is internal and Cisco NX-OS compares the MED to other
internal paths.
If the AS path starts with confederation segments followed by an AS_SEQUENCE, then the
peer autonomous system is the first AS number in the AS_SEQUENCE, and Cisco NX-OS
compares the MED to other paths that have the same peer autonomous system.
If Cisco NX-OS receives no MED attribute with the path, then Cisco NX-OS considers the
MED to be 0 unless you configure the best-path algorithm to set a missing MED to the
highest possible value. See the
more information.
If the nondeterministic MED comparison feature is enabled, the best path algorithm uses the
Cisco IOS style of MED comparison. See the
page 6-8 for more information.
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, NX-OS Release 5.0(3)U1(1)
"AS Confederations" section on page 6-4
"Tuning the Best-Path Algorithm" section on page 6-8
"Tuning the Best-Path Algorithm" section on
Information About Basic BGP
for more information.
"Tuning the Best-Path Algorithm"
for
5-5