5. Path Attributes
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories:
- Well-known mandatory.
- Well-known discretionary.
- Optional transitive.
- Optional non-transitive.
BGP implementations MUST recognize all well-known attributes. Some of these attributes are mandatory and MUST be included in every UPDATE message that contains NLRI. Others are discretionary and MAY or MAY NOT be sent in a particular UPDATE message.
Once a BGP peer has updated any well-known attributes, it MUST pass these attributes to its peers in any updates it transmits.
In addition to well-known attributes, each path MAY contain one or more optional attributes. It is not required or expected that all BGP implementations support all optional attributes. The handling of an unrecognized optional attribute is determined by the setting of the Transitive bit in the attribute flags octet. Paths with unrecognized transitive optional attributes SHOULD be accepted. If a path with an unrecognized transitive optional attribute is accepted and passed to other BGP peers, then the unrecognized transitive optional attribute of that path MUST be passed, along with the path, to other BGP peers with the Partial bit in the Attribute Flags octet set to 1. If a path with a recognized, transitive optional attribute is accepted and passed along to other BGP peers and the Partial bit in the Attribute Flags octet is set to 1 by some previous AS, it MUST NOT be set back to 0 by the current AS. Unrecognized non-transitive optional attributes MUST be quietly ignored and not passed along to other BGP peers.
New, transitive optional attributes MAY be attached to the path by the originator or by any other BGP speaker in the path. If they are not attached by the originator, the Partial bit in the Attribute Flags octet is set to 1. The rules for attaching new non-transitive optional attributes will depend on the nature of the specific attribute. The documentation of each new non-transitive optional attribute will be expected to include such rules (the description of the MULTI_EXIT_DISC attribute gives an example). All optional attributes (both transitive and non-transitive), MAY be updated (if appropriate) by BGP speakers in the path.
The sender of an UPDATE message SHOULD order path attributes within the UPDATE message in ascending order of attribute type. The receiver of an UPDATE message MUST be prepared to handle path attributes within UPDATE messages that are out of order.
The same attribute (attribute with the same type) cannot appear more than once within the Path Attributes field of a particular UPDATE message.
The mandatory category refers to an attribute that MUST be present in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE message. Attributes classified as optional for the purpose of the protocol extension mechanism may be purely discretionary, discretionary, required, or disallowed in certain contexts.
| attribute | EBGP | IBGP |
|---|---|---|
| ORIGIN | mandatory | mandatory |
| AS_PATH | mandatory | mandatory |
| NEXT_HOP | mandatory | mandatory |
| MULTI_EXIT_DISC | discretionary | discretionary |
| LOCAL_PREF | see Section 5.1.5 | required |
| ATOMIC_AGGREGATE | see Section 5.1.6 and 9.1.4 | |
| AGGREGATOR | discretionary | discretionary |