RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
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
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 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 ) 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.