Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.100.0 - Page 164

c AS_CONFED_SET is not included in the AS_PATH length. d 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: a 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. b If you entered the bgp always-compare-med command, MEDs are compared for all paths. c 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 Dell Networking OS deems the paths as equal and does not perform steps 9 through 11, if the following criteria is met: a the IBGP multipath or EBGP multipath are configured (the maximum-path command). b the paths being compared were received from the same AS with the same number of ASs in the AS Path but with different NextHops. c the paths were received from IBGP or EBGP neighbor respectively. 10 If the bgp bestpath router-id ignore command is enabled and: a if the Router-ID is the same for multiple paths (because the routes were received from the same route) skip this step. b 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 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 Dell Networking OS 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, Dell Networking OS compares MED between the adjacent paths within an AS group because all paths in the AS group are from the same AS. Weight The weight attribute is local to the router and is not advertised to neighboring routers. If the router learns about more than one route to the same destination, the route with the highest weight is preferred. The route with the highest weight is installed in the IP routing table. Local Preference Local preference (LOCAL_PREF) represents the degree of preference within the entire AS. The higher the number, the greater the preference for the route. Local preference (LOCAL_PREF) is one of the criteria used to determine the best path, so keep in mind that other criteria may impact selection, as shown in the illustration in Best Path Selection Criteria. For this example, assume that thelocal preference (LOCAL_PREF) is the only attribute applied. In the following illustration, AS100 has two possible paths to AS 200. Although the path through Router A is shorter (one hop instead of two), the LOCAL_PREF settings have the preferred path go through Router B and AS300. This is advertised to all routers within AS100, causing all BGP speakers to prefer the path through Router B. 164 Border Gateway Protocol IPv4 (BGPv4)