Multiprotocol Extensions for BGP-4
RFC 4760

Network Working Group T. Bates
Request for Comments: 4760 Cisco Systems
Obsoletes: 2858 R. Chandra
Category: Standards Track Sonoa Systems
 D. Katz
 Y. Rekhter
 Juniper Networks
 January 2007
 Multiprotocol Extensions for BGP-4
Status of This Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
 Copyright (C) The IETF Trust (2007).
Abstract
 This document defines extensions to BGP-4 to enable it to carry
 routing information for multiple Network Layer protocols (e.g., IPv6,
 IPX, L3VPN, etc.). The extensions are backward compatible - a router
 that supports the extensions can interoperate with a router that
 doesn't support the extensions.
Bates, et al. Standards Track [Page 1]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
1. Introduction
 The only three pieces of information carried by BGP-4 [BGP-4] that
 are IPv4 specific are (a) the NEXT_HOP attribute (expressed as an
 IPv4 address), (b) AGGREGATOR (contains an IPv4 address), and (c)
 NLRI (expressed as IPv4 address prefixes). This document assumes
 that any BGP speaker (including the one that supports multiprotocol
 capabilities defined in this document) has to have an IPv4 address
 (which will be used, among other things, in the AGGREGATOR
 attribute). Therefore, to enable BGP-4 to support routing for
 multiple Network Layer protocols, the only two things that have to be
 added to BGP-4 are (a) the ability to associate a particular Network
 Layer protocol with the next hop information, and (b) the ability to
 associate a particular Network Layer protocol with NLRI. To identify
 individual Network Layer protocols associated with the next hop
 information and semantics of NLRI, this document uses a combination
 of Address Family, as defined in [IANA-AF], and Subsequent Address
 Family (as described in this document).
 One could further observe that the next hop information (the
 information provided by the NEXT_HOP attribute) is meaningful (and
 necessary) only in conjunction with the advertisements of reachable
 destinations - in conjunction with the advertisements of unreachable
 destinations (withdrawing routes from service), the next hop
 information is meaningless. This suggests that the advertisement of
 reachable destinations should be grouped with the advertisement of
 the next hop to be used for these destinations, and that the
 advertisement of reachable destinations should be segregated from the
 advertisement of unreachable destinations.
 To provide backward compatibility, as well as to simplify
 introduction of the multiprotocol capabilities into BGP-4, this
 document uses two new attributes, Multiprotocol Reachable NLRI
 (MP_REACH_NLRI) and Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI).
 The first one (MP_REACH_NLRI) is used to carry the set of reachable
 destinations together with the next hop information to be used for
 forwarding to these destinations. The second one (MP_UNREACH_NLRI)
 is used to carry the set of unreachable destinations. Both of these
 attributes are optional and non-transitive. This way, a BGP speaker
 that doesn't support the multiprotocol capabilities will just ignore
 the information carried in these attributes and will not pass it to
 other BGP speakers.
2. Specification of Requirements
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].
Bates, et al. Standards Track [Page 2]
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].
Bates, et al. Standards Track [Page 3]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
 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.
Bates, et al. Standards Track [Page 4]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
 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.
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 <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.
 An UPDATE message that contains the MP_UNREACH_NLRI is not required
 to carry any other path attributes.
Bates, et al. Standards Track [Page 6]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
5. NLRI Encoding
 The Network Layer Reachability information is encoded as one or more
 2-tuples of the form <length, prefix>, whose fields are described
 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 address
 prefix. A length of zero indicates a prefix that matches all (as
 specified by the address family) addresses (with prefix, itself,
 of zero octets).
 b) Prefix:
 The Prefix field contains an address prefix followed by enough
 trailing bits to make the end of the field fall on an octet
 boundary. Note that the value of trailing bits is irrelevant.
6. Subsequent Address Family Identifier
 This document defines the following values for the Subsequent Address
 Family Identifier field carried in the MP_REACH_NLRI and
 MP_UNREACH_NLRI attributes:
 1 - Network Layer Reachability Information used for unicast
 forwarding
 2 - Network Layer Reachability Information used for multicast
 forwarding
 An implementation MAY support all, some, or none of the Subsequent
 Address Family Identifier values defined in this document.
Bates, et al. Standards Track [Page 7]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
7. Error Handling
 If a BGP speaker receives from a neighbor an UPDATE message that
 contains the MP_REACH_NLRI or MP_UNREACH_NLRI attribute, and if the
 speaker determines that the attribute is incorrect, the speaker MUST
 delete all the BGP routes received from that neighbor whose AFI/SAFI
 is the same as the one carried in the incorrect MP_REACH_NLRI or
 MP_UNREACH_NLRI attribute. For the duration of the BGP session over
 which the UPDATE message was received, the speaker then SHOULD ignore
 all the subsequent routes with that AFI/SAFI received over that
 session.
 In addition, the speaker MAY terminate the BGP session over which the
 UPDATE message was received. The session SHOULD be terminated with
 the Notification message code/subcode indicating "UPDATE Message
 Error"/"Optional Attribute Error".
8. Use of BGP Capability Advertisement
 A BGP speaker that uses Multiprotocol Extensions SHOULD use the
 Capability Advertisement procedures [BGP-CAP] to determine whether
 the speaker could use Multiprotocol Extensions with a particular
 peer.
 The fields in the Capabilities Optional Parameter are set as follows.
 The Capability Code field is set to 1 (which indicates Multiprotocol
 Extensions capabilities). The Capability Length field is set to 4.
 The Capability Value field is defined as:
 0 7 15 23 31
 +-------+-------+-------+-------+
 | AFI | Res. | SAFI |
 +-------+-------+-------+-------+
 The use and meaning of this field is as follow:
 AFI - Address Family Identifier (16 bit), encoded the same way as
 in the Multiprotocol Extensions
 Res. - Reserved (8 bit) field. SHOULD be set to 0 by the sender
 and ignored by the receiver.
 Note that not setting the field value to 0 may create issues
 for a receiver not ignoring the field. In addition, this
 definition is problematic if it is ever attempted to redefine
 the field.
Bates, et al. Standards Track [Page 8]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
 SAFI - Subsequent Address Family Identifier (8 bit), encoded the
 same way as in the Multiprotocol Extensions.
 A speaker that supports multiple <AFI, SAFI> tuples includes them as
 multiple Capabilities in the Capabilities Optional Parameter.
 To have a bi-directional exchange of routing information for a
 particular <AFI, SAFI> between a pair of BGP speakers, each such
 speaker MUST advertise to the other (via the Capability Advertisement
 mechanism) the capability to support that particular <AFI, SAFI>
 route.
9. IANA Considerations
 As specified in this document, the MP_REACH_NLRI and MP_UNREACH_NLRI
 attributes contain the Subsequence Address Family Identifier (SAFI)
 field. The SAFI name space is defined in this document. The IANA
 registered and maintains values for the SAFI namespace as follows:
 - SAFI values 1 and 2 are assigned in this document.
 - SAFI value 3 is reserved. It was assigned by RFC 2858 for a use
 that was never fully implemented, so it is deprecated by this
 document.
 - SAFI values 5 through 63 are to be assigned by IANA using either
 the Standards Action process, defined in [RFC2434], or the Early
 IANA Allocation process, defined in [RFC4020].
 - SAFI values 67 through 127 are to be assigned by IANA, using the
 "First Come First Served" policy, defined in RFC 2434.
 - SAFI values 0 and 255 are reserved.
 - SAFI values 128 through 240 are part of the previous "private
 use" range. At the time of approval of this document, the
 unused values were provided to IANA by the Routing Area
 Director. These unused values, namely, 130, 131, 135 through
 139, and 141 through 240, are considered reserved in order to
 avoid conflicts.
 - SAFI values 241 through 254 are for "private use", and values in
 this range are not to be assigned by IANA.
Bates, et al. Standards Track [Page 9]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
10. Comparison with RFC 2858
 This document makes the use of the next hop information consistent
 with the information carried in the NEXT_HOP BGP path attribute.
 This document removes the definition of SAFI 3 and deprecates SAFI 3.
 This document changes partitioning of the SAFI space. Specifically,
 in RFC 2858 SAFI values 128 through 240 were part of the "private
 use" range. This document specifies that of this range, allocations
 that are currently in use are to be recognized by IANA, and that
 unused values, namely 130, 131, 135 through 139, and 141 through 240,
 should be considered reserved.
 This document renames the Number of SNPAs field to Reserved and
 removes the rest of the SNPA-related information from the
 MP_REACH_NLRI attribute.
11. Comparison with RFC 2283
 This document restricts the MP_REACH_NLRI attribute to carry only a
 single instance of <AFI, SAFI, Next Hop Information, ...>.
 This document restricts the MP_UNREACH_NLRI attribute to carry only a
 single instance of <AFI, SAFI, ...>.
 This document clarifies handling of an UPDATE message that carries no
 NLRI, other than the one encoded in the MP_REACH_NLRI attribute.
 This document clarifies error handling in the presence of
 MP_REACH_NLRI or MP_UNREACH_NLRI attributes.
 This document specifies the use of BGP Capabilities Advertisements in
 conjunction with multi-protocol extensions.
 Finally, this document includes the "IANA Consideration" section.
12. Security Considerations
 This extension to BGP does not change the underlying security issues
 inherent in the existing BGP.
13. Acknowledgements
 The authors would like to thank members of the IDR Working Group for
 their review and comments.
Bates, et al. Standards Track [Page 10]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
14. Normative References
 [BGP-CAP] Chandra, R. and J. Scudder, "Capabilities Advertisement
 with BGP-4", RFC 3392, November 2002.
 [BGP-4] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
 Protocol 4 (BGP-4)", RFC 4271, January 2006.
 [IANA-AF] "Address Family Numbers", Reachable from
 http://www.iana.org/numbers.html
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
 IANA Considerations Section in RFCs", BCP 26, RFC 2434,
 October 1998.
 [RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of
 Standards Track Code Points", BCP 100, RFC 4020, February
 2005.
Authors' Addresses
 Tony Bates
 Cisco Systems, Inc.
 EMail: tbates@cisco.com
 Ravi Chandra
 Sonoa Systems
 EMail: rchandra@sonoasystems.com
 Dave Katz
 Juniper Networks, Inc.
 EMail: dkatz@juniper.net
 Yakov Rekhter
 Juniper Networks, Inc.
 EMail: yakov@juniper.net
Bates, et al. Standards Track [Page 11]
RFC 4760 Multiprotocol Extensions for BGP-4 January 2007
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Bates, et al. Standards Track [Page 12]