Section 5. 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.
   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
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:
         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
         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).
      2) 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.
      3) 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
           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
   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.
   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.