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5. Path Attributes

  1. Path Attributes

This section discusses the path attributes of the UPDATE message.

Path attributes fall into four separate categories:

  1. Well-known mandatory.
  2. Well-known discretionary.
  3. Optional transitive.
  4. Optional non-transitive.

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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.

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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

5.1. Path Attribute Usage

The usage of each BGP path attribute is described in the following clauses.

5.1.1. ORIGIN

ORIGIN is a well-known mandatory attribute. The ORIGIN attribute is generated by the speaker that originates the associated routing information. Its value SHOULD NOT be changed by any other speaker.

5.1.2. AS_PATH

AS_PATH is a well-known mandatory attribute. This attribute identifies the autonomous systems through which routing information carried in this UPDATE message has passed. The components of this list can be AS_SETs or AS_SEQUENCEs.

When a BGP speaker propagates a route it learned from another BGP speaker's UPDATE message, it modifies the route's AS_PATH attribute based on the location of the BGP speaker to which the route will be sent:

a) When a given BGP speaker advertises the route to an internal peer, the advertising speaker SHALL NOT modify the AS_PATH attribute associated with the route.

b) When a given BGP speaker advertises the route to an external peer, the advertising speaker updates the AS_PATH attribute as follows:

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  1. if the first path segment of the AS_PATH is of type AS_SEQUENCE, the local system prepends its own AS number as the last element of the sequence (put it in the leftmost position with respect to the position of octets in the protocol message). If the act of prepending will cause an overflow in the AS_PATH segment (i.e., more than 255 ASes), it SHOULD prepend a new segment of type AS_SEQUENCE and prepend its own AS number to this new segment.

  2. if the first path segment of the AS_PATH is of type AS_SET, the local system prepends a new path segment of type AS_SEQUENCE to the AS_PATH, including its own AS number in that segment.

  3. if the AS_PATH is empty, the local system creates a path segment of type AS_SEQUENCE, places its own AS into that segment, and places that segment into the AS_PATH.

When a BGP speaker originates a route then:

a) the originating speaker includes its own AS number in a path segment, of type AS_SEQUENCE, in the AS_PATH attribute of all UPDATE messages sent to an external peer. In this case, the AS number of the originating speaker's autonomous system will be the only entry the path segment, and this path segment will be the only segment in the AS_PATH attribute.

b) the originating speaker includes an empty AS_PATH attribute in all UPDATE messages sent to internal peers. (An empty AS_PATH attribute is one whose length field contains the value zero).

Whenever the modification of the AS_PATH attribute calls for including or prepending the AS number of the local system, the local system MAY include/prepend more than one instance of its own AS number in the AS_PATH attribute. This is controlled via local configuration.

5.1.3. NEXT_HOP

The NEXT_HOP is a well-known mandatory attribute that defines the IP address of the router that SHOULD be used as the next hop to the destinations listed in the UPDATE message. The NEXT_HOP attribute is calculated as follows:

  1. When sending a message to an internal peer, if the route is not locally originated, the BGP speaker SHOULD NOT modify the NEXT_HOP attribute unless it has been explicitly configured to announce its own IP address as the NEXT_HOP. When announcing a

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locally-originated route to an internal peer, the BGP speaker SHOULD use the interface address of the router through which the announced network is reachable for the speaker as the NEXT_HOP. If the route is directly connected to the speaker, or if the interface address of the router through which the announced network is reachable for the speaker is the internal peer's address, then the BGP speaker SHOULD use its own IP address for the NEXT_HOP attribute (the address of the interface that is used to reach the peer).

  1. When sending a message to an external peer, X, and the peer is one IP hop away from the speaker:

    • If the route being announced was learned from an internal peer or is locally originated, the BGP speaker can use an interface address of the internal peer router (or the internal router) through which the announced network is reachable for the speaker for the NEXT_HOP attribute, provided that peer X shares a common subnet with this address. This is a form of "third party" NEXT_HOP attribute.

    • Otherwise, if the route being announced was learned from an external peer, the speaker can use an IP address of any adjacent router (known from the received NEXT_HOP attribute) that the speaker itself uses for local route calculation in the NEXT_HOP attribute, provided that peer X shares a common subnet with this address. This is a second form of "third party" NEXT_HOP attribute.

    • Otherwise, if the external peer to which the route is being advertised shares a common subnet with one of the interfaces of the announcing BGP speaker, the speaker MAY use the IP address associated with such an interface in the NEXT_HOP attribute. This is known as a "first party" NEXT_HOP attribute.

    • By default (if none of the above conditions apply), the BGP speaker SHOULD use the IP address of the interface that the speaker uses to establish the BGP connection to peer X in the NEXT_HOP attribute.

  2. When sending a message to an external peer X, and the peer is multiple IP hops away from the speaker (aka "multihop EBGP"):

    • The speaker MAY be configured to propagate the NEXT_HOP attribute. In this case, when advertising a route that the speaker learned from one of its peers, the NEXT_HOP attribute of the advertised route is exactly the same as the NEXT_HOP

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attribute of the learned route (the speaker does not modify the NEXT_HOP attribute).

  • By default, the BGP speaker SHOULD use the IP address of the interface that the speaker uses in the NEXT_HOP attribute to establish the BGP connection to peer X.

Normally, the NEXT_HOP attribute is chosen such that the shortest available path will be taken. A BGP speaker MUST be able to support the disabling advertisement of third party NEXT_HOP attributes in order to handle imperfectly bridged media.

A route originated by a BGP speaker SHALL NOT be advertised to a peer using an address of that peer as NEXT_HOP. A BGP speaker SHALL NOT install a route with itself as the next hop.

The NEXT_HOP attribute is used by the BGP speaker to determine the actual outbound interface and immediate next-hop address that SHOULD be used to forward transit packets to the associated destinations.

The immediate next-hop address is determined by performing a recursive route lookup operation for the IP address in the NEXT_HOP attribute, using the contents of the Routing Table, selecting one entry if multiple entries of equal cost exist. The Routing Table entry that resolves the IP address in the NEXT_HOP attribute will always specify the outbound interface. If the entry specifies an attached subnet, but does not specify a next-hop address, then the address in the NEXT_HOP attribute SHOULD be used as the immediate next-hop address. If the entry also specifies the next-hop address, this address SHOULD be used as the immediate next-hop address for packet forwarding.

5.1.4. MULTI_EXIT_DISC

The MULTI_EXIT_DISC is an optional non-transitive attribute that is intended to be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four-octet unsigned number, called a metric. All other factors being equal, the exit point with the lower metric SHOULD be preferred. If received over EBGP, the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other BGP speakers within the same AS (see also 9.1.2.2). The MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be propagated to other neighboring ASes.

A BGP speaker MUST implement a mechanism (based on local configuration) that allows the MULTI_EXIT_DISC attribute to be removed from a route. If a BGP speaker is configured to remove the

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MULTI_EXIT_DISC attribute from a route, then this removal MUST be done prior to determining the degree of preference of the route and prior to performing route selection (Decision Process phases 1 and 2).

An implementation MAY also (based on local configuration) alter the value of the MULTI_EXIT_DISC attribute received over EBGP. If a BGP speaker is configured to alter the value of the MULTI_EXIT_DISC attribute received over EBGP, then altering the value MUST be done prior to determining the degree of preference of the route and prior to performing route selection (Decision Process phases 1 and 2). See Section 9.1.2.2 for necessary restrictions on this.

5.1.5. LOCAL_PREF

LOCAL_PREF is a well-known attribute that SHALL be included in all UPDATE messages that a given BGP speaker sends to other internal peers. A BGP speaker SHALL calculate the degree of preference for each external route based on the locally-configured policy, and include the degree of preference when advertising a route to its internal peers. The higher degree of preference MUST be preferred. A BGP speaker uses the degree of preference learned via LOCAL_PREF in its Decision Process (see Section 9.1.1).

A BGP speaker MUST NOT include this attribute in UPDATE messages it sends to external peers, except in the case of BGP Confederations [RFC3065]. If it is contained in an UPDATE message that is received from an external peer, then this attribute MUST be ignored by the receiving speaker, except in the case of BGP Confederations [RFC3065].

5.1.6. ATOMIC_AGGREGATE

ATOMIC_AGGREGATE is a well-known discretionary attribute.

When a BGP speaker aggregates several routes for the purpose of advertisement to a particular peer, the AS_PATH of the aggregated route normally includes an AS_SET formed from the set of ASes from which the aggregate was formed. In many cases, the network administrator can determine if the aggregate can safely be advertised without the AS_SET, and without forming route loops.

If an aggregate excludes at least some of the AS numbers present in the AS_PATH of the routes that are aggregated as a result of dropping the AS_SET, the aggregated route, when advertised to the peer, SHOULD include the ATOMIC_AGGREGATE attribute.

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A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute SHOULD NOT remove the attribute when propagating the route to other speakers.

A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute MUST NOT make any NLRI of that route more specific (as defined in 9.1.4) when advertising this route to other BGP speakers.

A BGP speaker that receives a route with the ATOMIC_AGGREGATE attribute needs to be aware of the fact that the actual path to destinations, as specified in the NLRI of the route, while having the loop-free property, may not be the path specified in the AS_PATH attribute of the route.

5.1.7. AGGREGATOR

AGGREGATOR is an optional transitive attribute, which MAY be included in updates that are formed by aggregation (see Section 9.2.2.2). A BGP speaker that performs route aggregation MAY add the AGGREGATOR attribute, which SHALL contain its own AS number and IP address. The IP address SHOULD be the same as the BGP Identifier of the speaker.

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