Dell Force10 Z9000 FTOS Configuration Guide for Z9000 System - Page 167

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. • Routes originated with the 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: • An AS_SET has a path length of 1, no matter how many ASs are in the set. • A path with no AS_PATH configured has a path length of 0. • AS_CONFED_SET is not included in the AS_PATH length. • 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. Border Gateway Protocol | 167