Network Working Group                                   Tony BatesInternet Draft                                       Cisco SystemsExpiration Date:  August 2000                         Ravi Chandra                                                     Siara Systems                                                         Dave Katz                                                  Juniper Networks                                                     Yakov Rekhter                                                     Cisco Systems                   Multiprotocol Extensions for BGP-4              draft-ietf-idr-bgp4-multiprotocol-v2-04.txt1. Status of this Memo   This document is an Internet-Draft and is in full conformance with   all provisions of Section 10 of RFC2026.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF), its areas, and its working groups.  Note that   other groups may also distribute working documents as Internet-   Drafts.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as ``work in progress.''   The list of current Internet-Drafts can be accessed at   http://www.ietf.org/ietf/1id-abstracts.txt   The list of Internet-Draft Shadow Directories can be accessed at   http://www.ietf.org/shadow.html.Bates, Chandra, Katz, Rekhter                                   [Page 1]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 20002. 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.3. 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 to   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 theBates, Chandra, Katz, Rekhter                                   [Page 2]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 2000   multiprotocol capabilities will just ignore the information carried   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 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 system   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)                                    |      +---------------------------------------------------------+Bates, Chandra, Katz, Rekhter                                   [Page 3]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 2000      | 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 in RFC1700 (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.      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 SNPABates, Chandra, Katz, Rekhter                                   [Page 4]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 2000         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 external   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 theBates, Chandra, Katz, Rekhter                                   [Page 5]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 2000   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.5. 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)                             |