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Network Working Group                                           T. BatesRequest for Comments: 2858                                    Y. RekhterObsoletes:2283                                            Cisco SystemsCategory: Standards Track                                     R. Chandra                                                    Redback Networks Inc                                                                 D. Katz                                                        Juniper Networks                                                               June 2000Multiprotocol Extensions for BGP-4Status 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 Internet Society (2000).  All Rights Reserved.Abstract   Currently BGP-4 [BGP-4] is capable of carrying routing information   only for IPv4 [IPv4]. This document defines extensions to BGP-4 to   enable it to carry routing information for multiple Network Layer   protocols (e.g., IPv6, IPX, etc...). The extensions are backward   compatible - a router that supports the extensions can interoperate   with a router that doesn't support the extensions.   This document obsoletesRFC 2283.1. Overview   The only three pieces of information carried by 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 toBates, et al.               Standards Track                     [Page 1]

RFC 2858           Multiprotocol Extensions for BGP-4          June 2000   associated a particular Network Layer protocol with NLRI. To identify   individual Network Layer protocols this document uses Address Family,   as defined in [RFC1700].   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. 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.      (c) to allow a given router to report some or all of the          Subnetwork Points of Attachment (SNPAs) that exist within the          local systemBates, 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 protocol         associated with the Network Address that follows. Presently         defined values for this field are specified inRFC 1700 (see         the Address Family Numbers section).      Subsequent Address Family Identifier:         This field provides additional information about the type of         the Network Layer Reachability Information carried in the         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 the         "Network Address of Next Hop" field as 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      Number of SNPAs:         A 1 octet field which contains the number of distinct SNPAs to         be listed in the following fields.  The value 0 may be used to         indicate that no SNPAs are listed in this attribute.      Length of Nth SNPA:         A 1 octet field whose value expresses the length of the "Nth         SNPA of Next Hop" field as measured in semi-octets      Nth SNPA of Next Hop:         A variable length field that contains an SNPA of the router         whose Network Address is contained in the "Network Address of         Next Hop" field.  The field length is an integral number of         octets in length, namely the rounded-up integer value of one         half the SNPA length expressed in semi-octets; if the SNPA         contains an odd number of semi-octets, a value in this field         will be padded with a trailing all-zero semi-octet.      Network Layer Reachability Information:         A variable length field that lists NLRI for the feasible routes         that are being advertised in this 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 border router that should be   used as the next hop to the destinations listed in the MP_NLRI   attribute in the UPDATE message.  When advertising a MP_REACH_NLRI   attribute to an external peer, a router may use one of its own   interface addresses in the next hop component of the attribute,   provided the external peer to which the route is being advertised   shares a common subnet with the next hop address.  This is known as a   "first party" next hop.  A BGP speaker can advertise to an externalBates, 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 hop   component, provided the external peer to which the route is being   advertised shares a common subnet with the next hop address.  This is   known as a "third party" next hop information.  A BGP speaker can   advertise any external peer router in the next hop component,   provided that the Network Layer address of this border router was   learned from an external peer, and the external peer to which the   route is being advertised shares a common subnet with the next hop   address.  This is a second form of "third party" next hop   information.   Normally the next hop information is chosen such that the shortest   available path will be taken.  A BGP speaker must be able to support   disabling advertisement of third party next hop information to handle   imperfectly bridged media or for reasons of policy.   A BGP speaker must never advertise an address of a peer to that peer   as a next hop, for a route that the speaker is originating.  A BGP   speaker must never install a route with itself as the next hop.   When a BGP speaker advertises the route to an internal peer, the   advertising speaker should not modify the next hop information   associated with the route.  When a BGP speaker receives the route via   an internal link, it may forward packets to the next hop address if   the address contained in the attribute is on a common subnet with the   local and remote BGP speakers.   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. If such a message is received from an   external peer, the local system shall check whether the leftmost AS   in the AS_PATH attribute is equal to the autonomous system number of   the peer than sent the message. If that is not the case, the local   system shall send the NOTIFICATION message with Error Code UPDATE   Message Error, and the Error Subcode set to Malformed AS_PATH.   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.3. 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.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 protocol         associated with the NLRI that follows. Presently defined values         for this field are specified inRFC 1700 (see the Address         Family Numbers section).      Subsequent Address Family Identifier:         This field provides additional information about the type of         the Network Layer Reachability Information carried in the         attribute.      Withdrawn Routes:         A variable length field that lists NLRI for the routes that are         being withdrawn from service. 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.4. 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)       |      +---------------------------+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 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.5. 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      3 - Network Layer Reachability Information used for both unicast          and multicast forwarding6. 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 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".Bates, et al.               Standards Track                     [Page 7]

RFC 2858           Multiprotocol Extensions for BGP-4          June 20007. 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:      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 way                as in the Multiprotocol Extensions         Res. - Reserved (8 bit) field. Should be set to 0 by the sender                and ignored by the receiver.         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>   routes.8. IANA Considerations   As specified in this document, the MPL_REACH_NLRI and MP_UNREACH_NLRI   attributes contain the Subsequence Address Family Identifier (SAFI)   field. The SAFI name space is defined inSection 9. The IANA will   maintain and register values for the SAFI namespace as follows.  SAFI   value 0 is reserved. SAFI values 1, 2, and 3 are assigned in this   document.  SAFI values 4 through 63 are to be assigned by IANA using   the "IETF Consensus" policy defined inRFC 2434. SAFI values 64   through 127 are to be assigned by IANA, using the "First Come FirstBates, et al.               Standards Track                     [Page 8]

RFC 2858           Multiprotocol Extensions for BGP-4          June 2000   Served" policy defined inRFC 2434. SAFI values 128 through 255 are   for "private use", and values in this range are not to be assigned by   IANA.9. Comparison withRFC 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.10. Security Considerations   This extension to BGP does not change the underlying security issues   inherent in the existing BGP [Heffernan].11. Acknowledgements   The authors would like to thank members of the IDR Working Group for   their review and comments.12. References   [BGP-CAP]    Chandra, R. and J. Scudder, "Capabilities Advertisement                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 TCP                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", STD                2,RFC 1700, October 1994. (see alsohttp://www.iana.org/iana/assignments.html)13. Authors' Addresses   Tony Bates   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, CA 95134   EMail: tbates@cisco.com   Ravi Chandra   Redback Networks Inc.   350, Holger Way   San Jose, CA 95134   EMail: rchandra@redback.com   Dave Katz   Juniper Networks, Inc.   3260 Jay St.   Santa Clara, CA 95054   EMail: dkatz@jnx.com   Yakov Rekhter   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, CA 95134   EMail: yakov@cisco.comBates, et al.               Standards Track                    [Page 10]

RFC 2858           Multiprotocol Extensions for BGP-4          June 200014.  Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Bates, et al.               Standards Track                    [Page 11]