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3. Multiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code 14)

This is an optional non-transitive attribute that can be used for the following purposes:

(a) to advertise a feasible route to a peer

(b) to permit a router to advertise the Network Layer address of the router that should be used as the next hop to the destinations listed in the Network Layer Reachability Information field of the MP_NLRI attribute.

The attribute is encoded as shown below:

    +---------------------------------------------------------+
    | Address Family Identifier (2 octets)                    |
    +---------------------------------------------------------+
    | Subsequent Address Family Identifier (1 octet)          |
    +---------------------------------------------------------+
    | Length of Next Hop Network Address (1 octet)            |
    +---------------------------------------------------------+
    | Network Address of Next Hop (variable)                  |
    +---------------------------------------------------------+
    | Reserved (1 octet)                                      |
    +---------------------------------------------------------+
    | Network Layer Reachability Information (variable)       |
    +---------------------------------------------------------+

The use and meaning of these fields are as follows:

Address Family Identifier (AFI):

This field in combination with the Subsequent Address Family Identifier field identifies the set of Network Layer protocols to which the address carried in the Next Hop field must belong, the way in which the address of the next hop is encoded, and the semantics of the Network Layer Reachability Information that follows. If the Next Hop is allowed to be from more than one Network Layer protocol, the encoding of the Next Hop MUST provide a way to determine its Network Layer protocol.

Presently defined values for the Address Family Identifier field are specified in the IANA's Address Family Numbers registry [IANA-AF].

Subsequent Address Family Identifier (SAFI):

This field in combination with the Address Family Identifier field identifies the set of Network Layer protocols to which the address carried in the Next Hop must belong, the way in which the address of the next hop is encoded, and the semantics of the Network Layer Reachability Information that follows. If the Next Hop is allowed to be from more than one Network Layer protocol, the encoding of the Next Hop MUST provide a way to determine its Network Layer protocol.

Length of Next Hop Network Address:

A 1-octet field whose value expresses the length of the "Network Address of Next Hop" field, measured in octets.

Network Address of Next Hop:

A variable-length field that contains the Network Address of the next router on the path to the destination system. The Network Layer protocol associated with the Network Address of the Next Hop is identified by a combination of <AFI, SAFI> carried in the attribute.

Reserved:

A 1 octet field that MUST be set to 0, and SHOULD be ignored upon receipt.

Network Layer Reachability Information (NLRI):

A variable length field that lists NLRI for the feasible routes that are being advertised in this attribute. The semantics of NLRI is identified by a combination of <AFI, SAFI> carried in the attribute.

When the Subsequent Address Family Identifier field is set to one of the values defined in this document, each NLRI is encoded as specified in the "NLRI encoding" section of this document.

The next hop information carried in the MP_REACH_NLRI path attribute defines the Network Layer address of the router that SHOULD be used as the next hop to the destinations listed in the MP_NLRI attribute in the UPDATE message.

The rules for the next hop information are the same as the rules for the information carried in the NEXT_HOP BGP attribute (see Section 5.1.3 of [BGP-4]).

An UPDATE message that carries the MP_REACH_NLRI MUST also carry the ORIGIN and the AS_PATH attributes (both in EBGP and in IBGP exchanges). Moreover, in IBGP exchanges such a message MUST also carry the LOCAL_PREF attribute.

An UPDATE message that carries no NLRI, other than the one encoded in the MP_REACH_NLRI attribute, SHOULD NOT carry the NEXT_HOP attribute. If such a message contains the NEXT_HOP attribute, the BGP speaker that receives the message SHOULD ignore this attribute.

An UPDATE message SHOULD NOT include the same address prefix (of the same <AFI, SAFI>) in more than one of the following fields: WITHDRAWN ROUTES field, Network Reachability Information fields, MP_REACH_NLRI field, and MP_UNREACH_NLRI field. The processing of an UPDATE message in this form is undefined.

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