Network Working Group Tony Bates Internet Draft Cisco Systems Expiration Date: December 1997 Ravi Chandra Cisco Systems Dave Katz Juniper Networks Yakov Rekhter Cisco Systems Multiprotocol Extensions for BGP-4 draft-bates-bgp4-multiprotocol-02.txt 1. Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 2. 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. Bates, Chandra, Katz, Rekhter [Page 1] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 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 the multiprotocol capabilities will just ignore the information carried in these attributes, and will not pass it to other BGP speakers. Bates, Chandra, Katz, Rekhter [Page 2] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 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 contains one or more triples
, where each triple is encoded as shown below: +---------------------------------------------------------+ | Address Family Identifier (2 octets) | +---------------------------------------------------------+ | Subsequent Address Familty 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 Length (2 octets)| +---------------------------------------------------------+ Bates, Chandra, Katz, Rekhter [Page 3] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 | 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. Presently defined values for this field are: 1 - Network Layer Reachability Information used for unicast forwarding 2 - Unicast Network Layer Reachability Information used for multicast forwarding 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 Bates, Chandra, Katz, Rekhter [Page 4] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 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 Length: This 2-octets unsigned integer indicates the total length of the Network Layer Reachability Information field in octets. 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 (see above), 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. Bates, Chandra, Katz, Rekhter [Page 5] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 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. When an UPDATE message carries the MP_REACH_NLRI attribute, the attribute shall be placed after all other attributes in the message. 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 contains one or more triples
, where each triple is encoded as shown below: +---------------------------------------------------------+ | Address Family Identifier (2 octets) | +---------------------------------------------------------+ | Subsequent Address Familty Identifier (1 octet) | +---------------------------------------------------------+ | Unfeasible Routes Length (2 octets) | +---------------------------------------------------------+ | 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 in RFC1700 (see the Address Family Bates, Chandra, Katz, Rekhter [Page 6] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 Numbers section). Subsequent Address Family Identifier: This field provides additional information about the type of the Network Layer Reachability Information carried in the attribute. Presently defined values for this field are: 1 - Network Layer Reachability Information used for unicast forwarding 2 - Unicast Network Layer Reachability Information used for multicast forwarding Unfeasible Routes Length: This 2-octets unsigned integer indicates the total length of the Withdrawn Routes field in octets. 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 (see above), 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. 6. NLRI encoding The Network Layer Reachability information is encoded as one or more 2-tuples of the form , whose fields are described below: +---------------------------+ | Length (1 octet) | +---------------------------+ | Prefix (variable) | +---------------------------+ The use and the meaning of these fields are as follows: Bates, Chandra, Katz, Rekhter [Page 7] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 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 address prefixes 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. 7. IPv6 Specific Issues A BGP speaker shall advertise to its peer in the Network Address of Next Hop field the global IPv6 address of the next hop, followed by the Link Local IPv6 address of the next hop if and only if the peer shares a common subnet with the entity identified by the global IPv6 address carried in the Network Address of Next Hop field (the value of the Length of Next Hop Network Address field shall be set to 32). In all other cases a BGP speaker shall advertise to its peer in the Network Address field only the global IPv6 address of the next hop (the value of the Length of Network Address of Next Hop field shall be set to 16). As a consequence, a BGP speaker that advertises a route to an internal peer may modify the Network Address of Next Hop field by removing the Link Local IPv6 address of the next hop. 8. Security Considerations Security issues are not discussed in this document. 9. Acknowledgements To be supplied. Bates, Chandra, Katz, Rekhter [Page 8] Internet Draft draft-bates-bgp4-multiprotocol-02.txt June 1997 10. References [BGP-4] [IPv4] [IPv6] [RFC1700] 11. Author Information Tony Bates Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 email: tbates@cisco.com Ravi Chandra Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 email: rchandra@cisco.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.com Bates, Chandra, Katz, Rekhter [Page 9]