Path attributes

BGP attaches path attributes (PAs) associated with each network path. The PAs provide BGP with granularity and control of routing policies within BGP. The BGP prefix PAs are classified as follows:

  • Well-known mandatory

  • Well-known discretionary

  • Optional transitive

  • Optional non-transitive

Per RFC 4271, well-known attributes must be recognized by all BGP implementations. Well-known mandatory attributes must be included with every prefix advertisement, while well-known discretionary attributes may or may not be included.

Optional attributes do not have to be recognized by all BGP implementations. Optional attributes can be set to be transitive and stay with the route advertisement from AS to AS. Other PAs are nontransitive and cannot be shared from AS to AS. BGP PAs are summarized in the following table.

Path Attribute

Description

Characteristics

AS_PATH

Lists ASNs through which the route has been advertised.

Well-known mandatory

NEXT_HOP

Lists the next-hop IP address used to reach an NLRI.

Well-known mandatory

LOCAL_PREF

Used to communicate a BGP router degree of preference for an advertised route. Used only in updates between internal BGP peers.

Well-known discretionary

MULTI_EXIT_DISc

Known as the MED, used to influence incoming traffic only in EBGP updates. Allows an AS to inform another AS of its preferred ingress point.

Optional non-transitive

AGGREGATOR

Lists the RouterId and ASN of the router that created the summary NLRI and indicates where the Aggregation occurred.

Optional transitive

ATOMIC_AGGREGATOR

Tags a summary NLRI as being a summary and indicates that a loss of path information has occurred.

Well-known discretionary

ORIGIN

Describes where the route was imported into BGP; i (IGP), e (EGP), or ? (incomplete information).

Well-known mandatory

COMMUNITY

Identifies a destination as a member of some community of destinations that share one or more common properties. Used to simplify policy enforcement.

Optional transitive

ORIGINATOR_ID

Used by route reflectors (RRs) to denote the router-id of the iBGP neighbor that injected the NLRI into the AS

Optional non-transitive

CLUSTER_LIST

Used by route reflectors (RRs) to list a sequence of route reflection cluster IDs through which the route has passed in order to prevent loops.

Optional non-transitive

MP_REACH_NLRI

Carries the set of reachable destinations together with the next hop information to be used for forwarding to these destinations.

Optional non-transitive

MP_UNREACH_NLRI

Used to withdraw unreachable routes.

Optional non-transitive