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10-5
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 10 Configuring Basic BGP
Information About Basic BGP
2. Cisco NX-OS chooses the path with the highest weight.
3. Cisco NX-OS chooses the path with the highest local preference.
4. If one of the paths is locally originated, Cisco NX-OS chooses that path.
5. Cisco NX-OS chooses the path with the shorter AS-path.
Note When calculating the length of the AS-path, Cisco NX-OS ignores confederation segments, and
counts AS sets as 1. See the “AS Confederations” section on page 11-4 for more information.
6. Cisco NX-OS chooses the path with the lower origin. Interior Gateway Protocol (IGP) is considered
lower than EGP.
7. Cisco NX-OS chooses the path with the lower multi- exit discriminator (MED).
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 “Tuning the Best-Path Algorithm”
section on page 11-9 for more information. Otherwise, Cisco NX-OS will perform a MED
comparison that depends on the AS-path attributes of the two paths being compared:
a. 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.
b. If the AS-path starts with an AS_SEQUENCE, then the peer autonomous system is the first AS
number in the sequence, andCisco NX-OS compares the MED to other paths that have the same
peer autonomous system.
c. 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.
d. 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.
Note 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 “Tuning the Best-Path Algorithm” section on page 11-9 for
more information.
e. If the nondeterministic MED comparison feature is enabled, the best path algorithm uses the
Cisco IOS style of MED comparison. See the “Tuning the Best-Path Algorithm” section on
page 11-9 for more information.
8. If one path is from an internal peer and the other path is from an external peer, then Cisco NX-OS
chooses the path from the external peer.
9. If the paths have different IGP metrics to their next-hop addresses, then Cisco NX-OS chooses the
path with the lower IGP metric.
10. Cisco NX-OS uses the path that was selected by the best-path algorithm the last time that it was run.
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