Dell S6000 FTOS 9.0(2.0) Configuration Guide for the System - Page 145

• 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: • 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. • If the bgp always-compare-med command is entered, MEDs are compared for all paths. • 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 next-hop is selected when synchronization is disabled and only an internal path remains. 9. FTOS deems the paths as equal and does not perform steps 9 through 11 listed below if the following criteria are met: • the IBGP multipath or EBGP multipath is configured (using the maximum-path command) • the bgp bestpath as-path multipath-relax command is not configured and the received paths are from the same AS with the same number of ASs in the AS path but have different NextHops • the bgp bestpath as-path multipath-relax command is configured and the learned paths are from different ASs but have equal AS path numbers • the paths were received from IBGP or EBGP neighbor respectively 10. If the bgp bestpath router-id ignore command is enabled and: • If the Router-ID is the same for multiple paths (because the routes were received from the same route) skip this step. • 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 should return without performing any of the checks outlined below. 11. Prefer the path originated from the BGP router with the lowest router ID. 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 is 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 FTOS choosing different best paths from a set of paths, depending on the order in which they were received from the neighbors since MED may or may not get compared between adjacent paths. In deterministic mode, FTOS compares MED between adjacent paths within an AS group since all paths in the AS group are from the same AS. Border Gateway Protocol | 123