RFC relevance of standards

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Network Working Group T.
Request for Comments: 2283 Cisco
Category: Standards Track R.
Cisco
D.
Juniper
Y.
Cisco
February 1998

Multiprotocol Extensions for BGP-4

Status of this

This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited

Copyright

Copyright (C) The Internet Society (1998). All Rights Reserved

2.

Currently BGP-4 [BGP-4] is capable of carrying routing
only for IPv4 [IPv4]. This document defines extensions to BGP-4
enable it to carry routing information for multiple Network
protocols (e.g., IPv6, IPX, etc...). The extensions are
compatible - a router that supports the extensions can
with a router that doesn't support the extensions

3.

The only three pieces of information carried by BGP-4 that are IPv
specific are (a) the NEXT_HOP attribute (expressed as an IPv
address), (b) AGGREGATOR (contains an IPv4 address), and (c)
(expressed as IPv4 address prefixes). This document assumes that
BGP speaker (including the one that supports
capabilities defined in this document) has to have an IPv4
(which will be used, among other things, in the
attribute). Therefore, to enable BGP-4 to support routing
multiple Network Layer protocols the only two things that have to
added to BGP-4 are (a) the ability to associate a particular
Layer protocol with the next hop information, and (b) the ability
associated a particular Network Layer protocol with NLRI. To

Bates, et. al. Standards Track [Page 1]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

individual Network Layer protocols this document uses Address Family
as defined in [RFC1700].

One could further observe that the next hop information (
information provided by the NEXT_HOP attribute) is meaningful (
necessary) only in conjunction with the advertisements of
destinations - in conjunction with the advertisements of
destinations (withdrawing routes from service) the next
information is meaningless. This suggests that the advertisement
reachable destinations should be grouped with the advertisement
the next hop to be used for these destinations, and that
advertisement of reachable destinations should be segregated from
advertisement of unreachable destinations

To provide backward compatibility, as well as to
introduction of the multiprotocol capabilities into BGP-4
document uses two new attributes, Multiprotocol Reachable
(MP_REACH_NLRI), and Multiprotocol Unreachable
(MP_UNREACH_NLRI). The first one (MP_REACH_NLRI) is used to carry
set of reachable destinations together with the next hop
to be used for forwarding to these destinations. The second
(MP_UNREACH_NLRI) is used to carry the set of
destinations. Both of these attributes are optional and non
transitive. This way a BGP speaker that doesn't support
multiprotocol capabilities will just ignore the information
in these attributes, and will not pass it to other BGP speakers

4. Multiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code 14):

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

(a) to advertise a feasible route to a

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

(c) to allow a given router to report some or all of
Subnetwork Points of Attachment (SNPAs) that exist within
local

The attribute contains one or more triples
Information, Next Hop Information, Network Layer
Information>, where each triple is encoded as shown below

Bates, et. al. Standards Track [Page 2]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

+---------------------------------------------------------+
| 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) |
+---------------------------------------------------------+
| Number of SNPAs (1 octet) |
+---------------------------------------------------------+
| Length of first SNPA(1 octet) |
+---------------------------------------------------------+
| First SNPA (variable) |
+---------------------------------------------------------+
| Length of second SNPA (1 octet) |
+---------------------------------------------------------+
| Second SNPA (variable) |
+---------------------------------------------------------+
| ... |
+---------------------------------------------------------+
| Length of Last SNPA (1 octet) |
+---------------------------------------------------------+
| Last SNPA (variable) |
+---------------------------------------------------------+
| Network Layer Reachability Information (variable) |
+---------------------------------------------------------+

The use and meaning of these fields are as follows

Address Family Identifier

This field carries the identity of the Network Layer
associated with the Network Address that follows.
defined values for this field are specified in RFC1700 (see
Address Family Numbers section).

Subsequent Address Family Identifier

This field provides additional information about the type
the Network Layer Reachability Information carried in
attribute

Length of Next Hop Network Address

A 1 octet field whose value expresses the length of
"Network Address of Next Hop" field as measured in

Bates, et. al. Standards Track [Page 3]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

Network Address of Next Hop

A variable length field that contains the Network Address
the next router on the path to the destination

Number of SNPAs

A 1 octet field which contains the number of distinct SNPAs
be listed in the following fields. The value 0 may be used
indicate that no SNPAs are listed in this attribute

Length of Nth SNPA

A 1 octet field whose value expresses the length of the "
SNPA of Next Hop" field as measured in semi-

Nth SNPA of Next Hop

A variable length field that contains an SNPA of the
whose Network Address is contained in the "Network Address
Next Hop" field. The field length is an integral number
octets in length, namely the rounded-up integer value of
half the SNPA length expressed in semi-octets; if the
contains an odd number of semi-octets, a value in this
will be padded with a trailing all-zero semi-octet

Network Layer Reachability Information

A variable length field that lists NLRI for the feasible
that are being advertised in this attribute. When
Subsequent Address Family Identifier field is set to one of
values defined in this document, each NLRI is encoded
specified in the "NLRI encoding" section of this document

The next hop information carried in the MP_REACH_NLRI path
defines the Network Layer address of the border router that should
used as the next hop to the destinations listed in the MP_
attribute in the UPDATE message. When advertising a MP_REACH_
attribute to an external peer, a router may use one of its
interface addresses in the next hop component of the attribute
provided the external peer to which the route is being
shares a common subnet with the next hop address. This is known as
"first party" next hop. A BGP speaker can advertise to an
peer an interface of any internal peer router in the next
component, provided the external peer to which the route is
advertised shares a common subnet with the next hop address. This
known as a "third party" next hop information. A BGP speaker
advertise any external peer router in the next hop component

Bates, et. al. Standards Track [Page 4]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

provided that the Network Layer address of this border router
learned from an external peer, and the external peer to which
route is being advertised shares a common subnet with the next
address. This is a second form of "third party" next
information

Normally the next hop information is chosen such that the
available path will be taken. A BGP speaker must be able to
disabling advertisement of third party next hop information to
imperfectly bridged media or for reasons of policy

A BGP speaker must never advertise an address of a peer to that
as a next hop, for a route that the speaker is originating. A
speaker must never install a route with itself as the next hop

When a BGP speaker advertises the route to an internal peer,
advertising speaker should not modify the next hop
associated with the route. When a BGP speaker receives the route
an internal link, it may forward packets to the next hop address
the address contained in the attribute is on a common subnet with
local and remote BGP speakers

An UPDATE message that carries the MP_REACH_NLRI must also carry
ORIGIN and the AS_PATH attributes (both in EBGP and in
exchanges). Moreover, in IBGP exchanges such a message must
carry the LOCAL_PREF attribute. If such a message is received from
external peer, the local system shall check whether the leftmost
in the AS_PATH attribute is equal to the autonomous system number
the peer than sent the message. If that is not the case, the
system shall send the NOTIFICATION message with Error Code
Message Error, and the Error Subcode set to Malformed AS_PATH

5. Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):

This is an optional non-transitive attribute that can be used for
purpose of withdrawing multiple unfeasible routes from service

The attribute contains one or more triples
Information, Unfeasible Routes Length, Withdrawn Routes>, where
triple is encoded as shown below

+---------------------------------------------------------+
| Address Family Identifier (2 octets) |
+---------------------------------------------------------+
| Subsequent Address Family Identifier (1 octet) |
+---------------------------------------------------------+
| Withdrawn Routes (variable) |
+---------------------------------------------------------+

Bates, et. al. Standards Track [Page 5]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

The use and the meaning of these fields are as follows

Address Family Identifier

This field carries the identity of the Network Layer
associated with the NLRI that follows. Presently defined
for this field are specified in RFC1700 (see the Address
Numbers section).

Subsequent Address Family Identifier

This field provides additional information about the type
the Network Layer Reachability Information carried in
attribute

Withdrawn Routes

A variable length field that lists NLRI for the routes that
being withdrawn from service. When the Subsequent
Family Identifier field is set to one of the values defined
this document, each NLRI is encoded as specified in the "
encoding" section of this document

An UPDATE message that contains the MP_UNREACH_NLRI is not
to carry any other path attributes

6. NLRI

The Network Layer Reachability information is encoded as one or
2-tuples of the form , whose fields are
below

+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+

The use and the meaning of these fields are as follows

a) Length

The Length field indicates the length in bits of the
prefix. A length of zero indicates a prefix that matches
(as specified by the address family) addresses (with prefix
itself, of zero octets).

Bates, et. al. Standards Track [Page 6]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

b) Prefix

The Prefix field contains address prefixes followed by
trailing bits to make the end of the field fall on an
boundary. Note that the value of trailing bits is irrelevant

7. Subsequent Address Family

This document defines the following values for the Subsequent
Family Identifier field carried in the MP_REACH_NLRI
MP_UNREACH_NLRI attributes

1 - Network Layer Reachability Information used for

2 - Network Layer Reachability Information used for

3 - Network Layer Reachability Information used for both
and multicast

This document reserves values 128-255 for vendor-
applications

This document reserves value 0.

Subsequent Address Family Identifiers (other than those reserved
vendor specific use) are assigned only by the IETF consensus
and IESG approval

8. Security

This extension to BGP does not change the underlying security issues

9.

The authors would like to thank members of the IDR Working Group
their review and comments

Bates, et. al. Standards Track [Page 7]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

10.

[BGP-4] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.

[IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.

[RFC1700] Reynolds, J., and J. Postel, "Assigned Numbers," STD 2,
RFC 1700, October 1994. (see
http://www.iana.org/iana/assignments.html

11. Author

Tony
Cisco Systems, Inc
170 West Tasman
San Jose, CA 95134

EMail: tbates@cisco.

Ravi
Cisco Systems, Inc
170 West Tasman
San Jose, CA 95134

EMail: rchandra@cisco.

Dave
Juniper Networks, Inc
3260 Jay St
Santa Clara, CA 95054

EMail: dkatz@jnx.

Yakov
Cisco Systems, Inc
170 West Tasman
San Jose, CA 95134

EMail: yakov@cisco.

Bates, et. al. Standards Track [Page 8]

RFC 2283 Multiprotocol Extensions for BGP-4 February 1998

12. Full Copyright

Copyright (C) The Internet Society (1998). All Rights Reserved

This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English

The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns

This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE

Bates, et. al. Standards Track [Page 9]

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