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AS-path attributes are used to provide route metrics. Along with the NLRI information are path attributes. Path Attributes allow BGP to make determinations of what is the best path. There are several categories that Path Attributes fall into:

AS-PATH ATTRIBUTE CATEGORIES

• Well-known, mandatory
• Well-known, discretionary
• Optional, Transitive
• Optional, Non-transitive

  WELL KNOWN, MANDATORY

  This attribute MUST appear in every UPDATE message. It must be supported by all BGP software implementations. If a well-known, mandatory attribute is missing from an UPDATE message, a NOTIFICATION message must be sent to the peer.

  Examples:

  • AS_path
  • ORIGIN
  • NEXT_HOP

  WELL KNOWN, DISCRETIONARY

  This attribute may or may not appear in an UPDATE message, but it MUST be supported by any BGP software implmentation.

  Examples:

  • LOCAL_PREF
  • ATOMIC_AGGREGATE

  OPTIONAL, TRANSITIVE

  These attributes may or may not be supported in all BGP implementations. If it is sent in an UPDATE message, but not recognized by the receiver, it should be passed on to the next AS.

  Examples:

  • AGGREGATOR
  • COMMUNITY

  OPTIONAL, NON-TRANSITIVE

  May or may not be supported, but if received, it is not required that the router pass it on. It may safely and quietly ignore the optional attribute.

  Examples:

  • MULTI_EXIT_DISC
  • ORIGINATOR_ID
  • Cluster List

 

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