Specifications

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Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
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Chapter 5 Configuring Basic BGP
Information About Basic BGP
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.
If all path parameters in Step 1 through Step 9 are the same, then you can configure the best-path
algorithm to compare the router IDs. See the “Tuning the Best-Path Algorithm” section on page 6-9 for
more information. If the path includes an originator attribute, then Cisco NX-OS uses that attribute as
the router ID to compare to; otherwise, Cisco NX-OS uses the router ID of the peer that sent the path. If
the paths have different router IDs, Cisco NX-OS chooses the path with the lower router ID.
Note When using the attribute originator as the router ID, it is possible that two paths have the same
router ID. It is also possible to have two BGP sessions with the same peer router, and therefore
you can receive two paths with the same router ID.
11. Cisco NX-OS selects the path with the shorter cluster length. If a path was not received with a cluster
list attribute, the cluster length is 0.
12. Cisco NX-OS chooses the path received from the peer with the lower IP address. Locally generated
paths (for example, redistributed paths) have a peer IP address of 0.
Note Paths that are equal after step 9 can be used for multipath if you configure multipath. See the “Load
Sharing and Multipath” section on page 6-6 for more information.
Step 2—Determining the Order of Comparisons
The second step of the BGP best-path algorithm implementation is to determine the order in which Cisco
NX-OS compares the paths:
1. Cisco NX-OS partitions the paths into groups. Within each group Cisco NX-OS compares the MED
among all paths. Cisco NX-OS uses the same rules as in the “Step 1—Comparing Pairs of Paths”
section on page 5-5 to determine whether MED can be compared between any two paths. Typically,
this comparison results in one group being chosen for each neighbor autonomous system. If you
configure the bgp bestpath med always command, then Cisco NX-OS chooses just one group that
contains all the paths.
2. Cisco NX-OS determines the best path in each group by iterating through all paths in the group and
keeping track of the best one so far. Cisco NX-OS compares each path with the temporary best path
found so far and if the new path is better, it becomes the new temporary best path and Cisco NX-OS
compares it with the next path in the group.
3. Cisco NX-OS forms a set of paths that contain the best path selected from each group in Step 2.
Cisco NX-OS selects the overall best path from this set of paths by going through them as in Step 2.
Step 3—Determining the Best-Path Change Suppression
The next part of the implementation is to determine whether Cisco NX-OS will use the new best path or
suppress the new best path. The router can continue to use the existing best path if the new one is
identical to the old path (if the router ID is the same). Cisco NX-OS continues to use the existing best
path to avoid route changes in the network.