Users Guide

Selection criteria
Best path selection criteria for BGP attributes:
1. Prefer the path with the largest WEIGHT attribute, and prefer the path with the largest LOCAL_PREF attribute.
2. Prefer the path that is locally originated using the network command, redistribute command, or aggregate-
address command. Routes originated using a network or redistribute command are preferred over routes that
originate with the aggregate-address command.
3. (Optional) If you configure the bgp bestpath as-path ignore command, skip this step because AS_PATH is not
considered. Prefer the path with the shortest AS_PATH:
● An AS_SET has a path length of 1 no matter how many 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
4. Prefer the path with the lowest ORIGIN type—IGP is lower than EGP and EGP is lower than INCOMPLETE.
5. Prefer the path with the lowest multiexit discriminator (MED) attribute:
● This comparison is only done if the first neighboring AS is the same in the two paths. The MEDs compare only if the first
AS in the AS_SEQUENCE is the same for both paths.
● Configure the bgp always-compare-med command to compare MEDs for all paths.
● Paths with no MED are treated as “worst” and assigned a MED of 4294967295.
6. Prefer external (EBGP) to internal (IBGP) paths or confederation EBGP paths, and prefer the path with the lowest IGP
metric to the BGP next-hop.
7. The system deems the paths as equal and only performs the following steps if the criteria are not met:
● Configure the IBGP multipath or EBGP multipath using the maximum-path command.
● The paths being compared were received from the same AS with the same number of AS in the AS Path but with
different next-hops.
● The paths were received from IBGP or EBGP neighbor, respectively.
8. If you enable the bgp bestpath router-id ignore command 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 returns without performing any of the checks detailed.
9. 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 an RR attribute, the originator ID is substituted for the router ID. 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.
10. 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.
In Non-Deterministic mode, the bgp non-deterministic-med command applies. Paths compare in the order they arrive.
This method leads to system selection of different best paths from a set of paths. Depending on the order they were received
from the neighbors, MED may or may not get compared between the adjacent paths. In Deterministic mode, the system
compares MED. MED is compared between the adjacent paths within an AS group because all paths in the AS group are from
the same AS.
Weight and local preference
The weight attribute is local to the router and does not advertise 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.
The 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_PREF is one of the criteria that determines the best path — other criteria may impact selection, see Best path
selection. Assume that LOCAL_PREF is the only attribute applied and AS 100 has two possible paths to AS 200. Although the
path through Router A is shorter, the LOCAL_PREF settings have the preferred path going through Router B and AS 300. This
advertises to all routers within AS 100, causing all BGP speakers to prefer the path through Router B.
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