Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

The best path in each group is selected based on specific criteria. Only one “best path” is selected at a time. If any of the criteria results in more than one path, BGP moves on to the next option in the list. For example, two paths may have the same weights, but different local preferences. BGP sees that the Weight criteria results in two potential “best paths” and moves to local preference to reduce the options. If a number of best paths are 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 the system 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, the system compares MED between the adjacent paths within an AS group because all paths in the AS group are from the same AS.

Best Path Selection Details

1. Prefer the path with the largest WEIGHT attribute.
2. Prefer the path with the largest LOCAL_PREF attribute.
3. Prefer the path that was locally Originated via a network command, redistribute command or aggregate-address command.
   1. Routes originated with the Originated via a network or redistribute commands are preferred over routes originated with the aggregate-address command.
4. Prefer the path with the shortest AS_PATH (unless the bgp bestpath as-path ignore command is configured, then AS_PATH is not considered). The following criteria apply:
   1. An AS_SET has a path length of 1, no matter how many ASs are in the set.
   2. A path with no AS_PATH configured has a path length of 0.
   3. AS_CONFED_SET is not included in the AS_PATH length.
   4. 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:
   1. 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.
   2. If you entered the bgp always-compare-med command, MEDs are compared for all paths.
   3. 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. The system deems the paths as equal and does not perform steps 9 through 11, if the following criteria is met:
   1. the IBGP multipath or EBGP multipath are configured (the maximum-path command).
   2. the paths being compared were received from the same AS with the same number of ASs in the AS Path but with different NextHops.
   3. the paths were received from IBGP or EBGP neighbor respectively.
10. If the bgp bestpath router-id ignore command is enabled and:
    1. if the Router-ID is the same for multiple paths (because the routes were received from the same route) skip this step.
    2. 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 are 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 the system 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, the system compares MED between the adjacent paths within an AS group because all paths in the AS group are from the same AS.