Reference Guide

Attributes
Routes learned using BGP have associated properties that are used to determine the best route to a destination when multiple
paths exist to a particular destination. These properties are called BGP attributes which influence route selection for designing
robust networks. There are no hard-coded limits on the number of supported BGP attributes.
BGP attributes for route selection:
● Weight
● Local preference
● Multiexit discriminators
● Origin
● AS path
● Next-hop
Communities
BGP communities are sets of routes with one or more common attributes. Communities assign common attributes to multiple
routes at the same time. Duplicate communities are not rejected.
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 the 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.
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