As per Relevance of the word extension, we have this rfc below:
Network Working Group T.
Request for Comments: 2858 Y.
Obsoletes: 2283 Cisco
Category: Standards Track R.
Redback Networks
D.
Juniper
June 2000
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 (2000). All Rights Reserved
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
This document obsoletes RFC 2283.
1.
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
Bates, et al. Standards Track [Page 1]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
associated a particular Network Layer protocol with NLRI. To
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
2. 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
destinations listed in the Network Layer
Information field of the MP_NLRI attribute
(c) to allow a given router to report some or all of
Subnetwork Points of Attachment (SNPAs) that exist within
local
Bates, et al. Standards Track [Page 2]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
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) |
+---------------------------------------------------------+
| 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 RFC 1700 (
the Address Family Numbers section).
Subsequent Address Family Identifier
This field provides additional information about the type
the Network Layer Reachability Information carried in
attribute
Bates, et al. Standards Track [Page 3]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
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
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
Bates, et al. Standards Track [Page 4]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
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
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
An UPDATE message that carries no NLRI, other than the one encoded
the MP_REACH_NLRI attribute, should not carry the NEXT_HOP attribute
If such a message contains the NEXT_HOP attribute, the BGP
that receives the message should ignore this attribute
3. 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
Bates, et al. Standards Track [Page 5]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
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
This field carries the identity of the Network Layer
associated with the NLRI that follows. Presently defined
for this field are specified in RFC 1700 (see the
Family 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
4. 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) |
+---------------------------+
Bates, et al. Standards Track [Page 6]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
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).
b) Prefix
The Prefix field contains an address prefix followed by
trailing bits to make the end of the field fall on an
boundary. Note that the value of trailing bits is irrelevant
5. 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
6. Error
If a BGP speaker receives from a neighbor an Update message
contains the MP_REACH_NLRI or MP_UNREACH_NLRI attribute, and
speaker determines that the attribute is incorrect, the speaker
delete all the BGP routes received from that neighbor whose AFI/
is the same as the one carried in the incorrect MP_REACH_NLRI
MP_UNREACH_NLRI attribute. For the duration of the BGP session
which the Update message was received, the speaker then should
all the subsequent routes with that AFI/SAFI received over
session
In addition, the speaker may terminate the BGP session over which
Update message was received. The session should be terminated
the Notification message code/subcode indicating "Update
Error"/"Optional Attribute Error".
Bates, et al. Standards Track [Page 7]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
7. Use of BGP Capability
A BGP speaker that uses Multiprotocol Extensions should use
Capability Advertisement procedures [BGP-CAP] to determine
the speaker could use Multiprotocol Extensions with a
peer
The fields in the Capabilities Optional Parameter are set as follows
The Capability Code field is set to 1 (which indicates
Extensions capabilities). The Capability Length field is set to 4.
The Capability Value field is defined as
The use and meaning of this field is as follow
0 7 15 23 31
+-------+-------+-------+-------+
| AFI | Res. | SAFI |
+-------+-------+-------+-------+
AFI - Address Family Identifier (16 bit), encoded the same
as in the Multiprotocol
Res. - Reserved (8 bit) field. Should be set to 0 by the
and ignored by the receiver
SAFI - Subsequent Address Family Identifier (8 bit),
the same way as in the Multiprotocol Extensions
A speaker that supports multiple tuples includes them
multiple Capabilities in the Capabilities Optional Parameter
To have a bi-directional exchange of routing information for
particular between a pair of BGP speakers, each
speaker must advertise to the other (via the Capability
mechanism) the capability to support that particular
routes
8. IANA
As specified in this document, the MPL_REACH_NLRI and MP_UNREACH_
attributes contain the Subsequence Address Family Identifier (SAFI
field. The SAFI name space is defined in Section 9. The IANA
maintain and register values for the SAFI namespace as follows.
value 0 is reserved. SAFI values 1, 2, and 3 are assigned in
document. SAFI values 4 through 63 are to be assigned by IANA
the "IETF Consensus" policy defined in RFC 2434. SAFI values 64
through 127 are to be assigned by IANA, using the "First Come
Bates, et al. Standards Track [Page 8]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
Served" policy defined in RFC 2434. SAFI values 128 through 255
for "private use", and values in this range are not to be assigned
IANA
9. Comparison with RFC 2283
This document restricts the MP_REACH_NLRI attribute to carry only
single instance of Information, ...>.
This document restricts the MP_UNREACH_NLRI attribute to carry only
single instance of .
This document clarifies handling of an UPDATE message that carries
NLRI, other than the one encoded in the MP_REACH_NLRI attribute
This document clarifies error handling in the presence
MP_REACH_NLRI or MP_UNREACH_NLRI attributes
This document specifies the use of BGP Capabilities Advertisements
conjunction with Multi-protocol extensions
Finally, this document includes the "IANA Consideration" Section
10. Security
This extension to BGP does not change the underlying security
inherent in the existing BGP [Heffernan].
11.
The authors would like to thank members of the IDR Working Group
their review and comments
12.
[BGP-CAP] Chandra, R. and J. Scudder, "Capabilities
with BGP-4", RFC 2842, May 2000.
[BGP-4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[Heffernan] Heffernan, A., "Protection of BGP Sessions via the
MD5 Signature Option", RFC 2385, August 1998.
[IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
Bates, et al. Standards Track [Page 9]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
[RFC1700] Postel, J. and J. K. Reynolds, "Assigned Numbers",
2, RFC 1700, October 1994. (see
http://www.iana.org/iana/assignments.html
13. Authors'
Tony
Cisco Systems, Inc
170 West Tasman
San Jose, CA 95134
EMail: tbates@cisco.
Ravi
Redback Networks Inc
350, Holger
San Jose, CA 95134
EMail: rchandra@redback.
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 10]
RFC 2858 Multiprotocol Extensions for BGP-4 June 2000
14. Full Copyright
Copyright (C) The Internet Society (2000). 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
Funding for the RFC Editor function is currently provided by
Internet Society
Bates, et al. Standards Track [Page 11]
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