4. Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):
This is an optional non-transitive attribute that can be used for the
purpose of withdrawing multiple unfeasible routes from service.
The attribute is encoded as shown below:
+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Withdrawn Routes (variable) |
+---------------------------------------------------------+
The use and the 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.
Bates, et al. Standards Track [Page 5]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
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.
Withdrawn Routes Network Layer Reachability Information:
A variable-length field that lists NLRI for the routes that are
being withdrawn from service. 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.
An UPDATE message that contains the MP_UNREACH_NLRI is not required
to carry any other path attributes.