Note
If Cisco NX-OS receives no MED attribute with the path, 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.
e. If the nondeterministic MED comparison feature is enabled, the best-path algorithm uses the Cisco IOS style of MED comparison.
8. If one path is from an internal peer and the other path is from an external peer, Cisco NX-OS chooses the path from the external peer.
9. If the paths have different IGP metrics to their next-hop addresses, 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, you can configure the best-path algorithm to compare the router IDs. If the path includes an originator attribute, 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 the attribute originator is used 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 it.
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 step 1 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, 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 it. 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.
You can turn off the suppression feature by configuring the best-path algorithm to compare the router IDs. If you configure this feature, the new best path is always preferred to the existing one.
You cannot suppress the best-path change if any of the following conditions occur:
The existing best path is no longer valid.
Either the existing or new best paths were received from internal (or confederation) peers or were locally generated (for example, by redistribution).
The paths were received from the same peer (the paths have the same router ID).
The paths have different weights, local preferences, origins, or IGP metrics to their next-hop addresses.
The paths have different MEDs.
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