Network Working Group Tony Przygienda (Redback) Internet Draft Naiming Shen (Redback) Nischal Sheth (Juniper) June 2002 Expires: December 2002 M-ISIS: Multi Topology (MT) Routing in IS-IS 1. 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. 2. Abstract This draft describes an optional mechanism within ISIS used today by many ISPs for IGP routing within their clouds. This draft describes how to run within a single ISIS domain a set of independent IP topologies that we call Multi-Topologies (MTs). This MT extension can be used for variety of purposes such as an in-band management network ``on top'' of the original IGP topology, maintain separate IGP routing domains for isolated multicast or IPv6 islands within the backbone, or force a subset of an address space to follow a different topology. 3. 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]. Przygienda, Shen, Sheth Expires December 2002 [Page 1] Internet Draft M-ISIS June 2002 4. Introduction Maintaining multiple MTs for ISIS [ISO10589 , RFC1195] in a backwards-compatible manner necessitates several extensions to the packet encoding and additional SPF procedures. The problem can be partitioned into forming of adjacencies, and advertising of prefixes and reachable intermediate systems within each topology. Having put all the necessary additional information in place, it must be properly used by MT capable SPF computation. The following sections describe each of the problems separately. To simplify the text, ``standard'' ISIS topology is defined to be MT ID #0 (zero). 5. Maintaining MT Adjacencies Each adjacency formed MUST be classified as belonging to a set of MTs on the interface. This is achieved by adding a new TLV into IIH packets that advertises which topologies the interface belongs to. If MT #0 is the only MT on the interface, it is optional to advertise it in the new TLV. Thus not including such a TLV in the IIH implies MT ID #0 capability only. Through this exchange of MT capabilities, a router is able to advertise the IS TLVs in LSPs with common MT set over those adjacencies. As a simplifying side-effect, boundaries between levels will be the same for all MTs. In the case of adjacency contains multiple MTs on an interface, and if there exists overlapping IP address space among the topologies, additional mechanism MAY be used to resolve the topology identity of the incoming IP packets on the interface. 5.1. Forming Adjacencies on Point-to-Point Interfaces Adjacencies on point-to-point interfaces are formed as usual with ISIS routers not implementing MT extensions. If local router does not participate in certain MTs, it will not advertise those MTIDs in it's IIHs and thus will not include that neighbor within it's topology based LSPs. On the other hand, if a MTID is not detected in remote side's IIHs, the local router MUST NOT include that neighbor within it's MT LSPs. The local router SHOULD NOT form adjacency if they don't have at least one common MT set over the interface. 5.2. Forming Adjacencies on Broadcast Interfaces On a LAN, all the routers on the LAN which implement the MT extension MAY advertise their MT capability TLV in their IIHs. If there is at least one adjacency on the LAN interface which belongs to this MT, the MT capable router MUST include according MT IS Reachable TLV in its LSP, otherwise it MAY include this MT Przygienda, Shen, Sheth Expires December 2002 [Page 2] Internet Draft M-ISIS June 2002 IS Reachable TLV in it's LSP if the LAN interface participates in this MT set. Two Routers on a LAN SHALL always establish adjacency regardless whether they have common MT set or not. This is to ensure all the routers on the LAN can correctly elect the same DIS. The IS SHOULD NOT include the MT IS TLV in its LSP if none of the adjacencies on the LAN contains this MT. The DIS, CSNP and PSNP functions are not changed by MT extension. 6. Advertising MT Reachable Intermediate Systems in LSPs A router MUST include within its LSPs in the Reachable Intermediate Systems TLVs only adjacent nodes that are participating in the according topology and advertise such TLVs only if it participates itself in the according topology. Standard Reachable Intermediate Systems TLV is acting here as MT ID #0 equivalent of the newly introduced MT Reachable Intermediate Systems TLV. A router MUST announce the MT IS TLV when there is at least one adjacency on the interface that belongs to this MT, otherwise it MAY announce the MT IS TLV of an adjacency for a given MT if this interface participates in the LAN. Since it is not possible to prevent a router that does not understand MT extensions from being responsible for generation of the according pseudo-node, it is not possible either to introduce special TLVs in the pseudo-node LSPs nor run distinct DIS elections per MT. Therefore, a generated pseudo-node LSP by DIS MUST contain in its IS Reachable TLV all nodes on the LAN as usual regardless of their MT capabilities. In other words, there is no change to the pseudo-node LSP construction. 7. MTs and Overload, Partition and Attached Bits As stated earlier, all MTs share the same set of L1-L2 boundaries and NETs. However, a router could for each of the MTs become potentially partitioned, overloaded and attached independently. To prevent unnecessary complexity, MT extensions does not support partition repair. Besides that, overload bit is assumed to be shared for all topologies. Attached bit is part of the MT TLV being distributed within a node's LSP fragment 0. Since each adjacency can belong to different MTs, it is possible that some MTs are L2 attached, and others are not on the same router. 8. Advertising MT Specific IP Prefixes Each of the MTs commands its own address space so a new TLV is necessary for prefixes stored in MTs other than MT ID #0. To Przygienda, Shen, Sheth Expires December 2002 [Page 3] Internet Draft M-ISIS June 2002 make the encoding less confusing when same prefixes are present in multiple MTs and accelerate SPF per MT, rather than adding a sub-TLV in TE extensions, a new TLV is introduced for that purpose that closely follows TE encoding [LS01]. 9. MT SPF Computation Each MT MUST run its own instance of the decision process. The LSP overload bit and pseudo-node LSPs are used by all topologies during computation. Reverse connectivity check within SPF MUST follow the according MT to assure the bi-directional reachability. The results of each computation MUST be stored in a separate RIB, otherwise overlapping addresses in different topologies could lead to undesirable routing behavior such as forwarding loops. The forwarding logic and configuration need to ensure the same MT is traversed from the source to the destination for packets. The nexthops derived from the MT SPF MUST belong to the adjacencies conforming to the same MT for correct forwarding. 9.1 Generic MT SPF (GSPF) for Network Management When multiple ISIS topologies exist within a domain, some of the routers can be configured to participate in a subset of the MTs in the network. For example, some routers can be IPv4 multicast MT only. Since each MT SPF installs the routes exclusively into the RIB belonging to their corresponding topology, there may not exist a routing path to reach some of those routers from the network management or troubleshooting point of view. To allow for management access to all routers irregardless of their MT configuration, we introduce here the concept of MT Management Colored Prefix and Generic MT SPF. A MT Management Colored Prefix is a MT IPv4 Reachable Prefix attached with the MT Management Prefix Color sub-TLV in LSPs. For example, a loopback interface IP adress of a MT based router can be configured with this MT Management Prefix Color to allow network management access to this router. The optional Generic MT SPF is an ISIS decision process that runs SPF algorithm with all the LSPs of routers which support Generic MT SPF, and it understands all the Reachable IS TLVs regardless of their MT property (including MT ID #0). It searches through all the LSPs for MT based Reachable IPv4 Prefixes TLV with the MT Management Color and installs them into the ``standard'' or ``management'' RIB during the Generic MT SPF. The router's Generic MT SPF capability is indicated with a bit inside all the MT capability TLVs within the node LSP fragment 0. The reverse connectivity check of Generic MT SPF MUST match the same MT between the two nodes. Implementations SHOULD allow the administrator to control the setting of MT Management Color to specific prefixes of the router Przygienda, Shen, Sheth Expires December 2002 [Page 4] Internet Draft M-ISIS June 2002 and the setting of Generic MT SPF to install the MT Mangement colored routes into specific RIB. It is recommended for the administrator's to ensure consistent configuration of all routers in the domain to prevent undesirable forwarding behavior. 10. LSP Flooding The LSP flooding mechanism is not changed by this MT extension. An implementation MAY have the option to use additional MT information in the LSP and on the adjacency to reduce some of the unnecessary LSP flooding over the point-to-point links. If a receiving interface and an outgoing interface don't share any common MT set, the implementation MAY have the option not to flood this LSP out on that interface. Since the fragment zero LSP contains the MTID, the MT capability of any LSP can be identified. If the LSP and the adjacencies of an outgoing interface do not share any common MT capability, the implementation MAY have the option not to flood this LSP out on that interface. An implementation MAY want to have the operators to control those optimization base on network topology and environment to ensure the LSP flooding reliability. For the neighbors have the Generic MT SPF enabled, this flooding optimization SHOULD NOT be used over the links to those neighbors. When there is an adjacency MT set change over an point-to-point link and the adjacency is in UP state, both sides SHOULD force an exchange of one set of CSNPs over that link. 11. Packet Encoding Three new TLVs are added to support MT extensions. One of them is common for the LSPs and IIHs. Encoding of Intermediate System TLV and IPv4 Reachable Prefixes is tied to traffic engineering extensions [LS01] to simplify the implementation effort. The main reasons we choose using new TLVs instead of using sub-TLVs inside existing TLV type-22 and type-135 are: In many cases, multi-topologies are non-congruent, using sub-TLV approach will not save LSP space; Many sub-TLVs are already being used in TLV type-22, and many more are being proposed while there is a maximum limit on the TLV size, from the existing TLVs; If traffic engineering or some other applications are being applied per topology level later, the new TLVs can automatically inherit the same attributes already defined for the ``standard'' IPv4 topology without going through long standard process to redefine them per topology. Przygienda, Shen, Sheth Expires December 2002 [Page 5] Internet Draft M-ISIS June 2002 11.1. Multi-Topology TLV TLV number of this TLV is 229. It contains one or more MTs the router is participating in the following structure: x CODE - 229 x LENGTH - total length of the value field, it should be 2 times the number of MT components. x VALUE - one or more 2-byte MT components, structured as follows: No. of Octets +--------------------------------+ |G |A |R |R | MT ID | 2 +--------------------------------+ Bits R are reserved, should be set to 0 on transmission and ignored on receipt. Bit G represents the router Generic MT SPF (only valid in LSP fragment #0, otherwise should be set to 0 on transmission and ignored on receipt.) If the router is configured for GSPF, all the MT TLV occurrences in the LSP MUST set this G bit; if any of the MT TLV has this G bit cleared, the router will be completely disregarded during GSPF computation. Bit A represents the ATTACH bit for the MT (only valid in LSP fragment #0 for MTs other than ID #0, otherwise should be set to 0 on transmission and ignored on receipt.) MT ID is a 12-bit field containing the ID of the topology being announced. This MT TLV can advertise up to 127 MTs and it can occur multiple times if needed within IIHs and LSP fragment 0. The result MT set should be the union of all the MT TLV occurrence in the packet. Any other ISIS PDU occurrence of this TLV MUST be ignored. Lack of MT TLV in hellos and fragment 0 LSP MUST be interpreted as participation of the advertising interface or router in MT ID #0 only. If a router advertises MT TLV, it has to advertise all the MTs it participates in, specifically including topology ID #0 also. 11.2. MT Intermediate Systems TLV TLV number of this TLV is 222. It is aligned with extended IS reachability TLV type 22 beside an additional two bytes in front at the beginning of the TLV. x CODE - 222 x LENGTH - total length of the value field Przygienda, Shen, Sheth Expires December 2002 [Page 6] Internet Draft M-ISIS June 2002 x VALUE - 2-byte MT membership plus the format of extended IS reachability TLV, structured as follows: No. of Octets +--------------------------------+ |R |R |R |R | MT ID | 2 +--------------------------------+ | extended IS TLV format | 11 - 253 +--------------------------------+ . . . . +--------------------------------+ | extended IS TLV format | 11 - 253 +--------------------------------+ Bits R are reserved, should be set to 0 on transmission and ignored on receipt. MT ID is a 12-bit field containing the ID of the topology being announced. After the 2-byte MT membership format, the MT IS content is in the same format as extended IS TLV, type 22 [LS01]. It can contain up to 23 neighbors of the same MT if no sub-TLVs are used. This TLV can occur multiple times. 11.3. Multi-Topology Reachable IPv4 Prefixes TLV TLV number of this TLV is 235. It is aligned with extended IS reachability TLV type 135 beside an additional two bytes in front. x CODE - 235 x LENGTH - total length of the value field x VALUE - 2-byte MT membership plus the format of extended extended IP reachability TLV, structured as follows: No. of Octets +--------------------------------+ |R |R |R |R | MT ID | 2 +--------------------------------+ | extended IP TLV format | 5 - 253 +--------------------------------+ . . . . +--------------------------------+ | extended IP TLV format | 5 - 253 +--------------------------------+ Bits R are reserved, should be set to 0 on transmission Przygienda, Shen, Sheth Expires December 2002 [Page 7] Internet Draft M-ISIS June 2002 and ignored on receipt. MT ID is a 12-bit field containing the ID of the topology being announced. After the 2-byte MT membership format, the MT IPv4 content is in the same format as extended IP reachability TLV, type 135 [LS01]. This TLV can occur multiple times. A new sub-TLV for this MT Reachable IPv4 Prefixes TLV is proposed: x Sub-TLV CODE - 117 x LENGTH - 4 octets x VALUE - The value of 1 of this sub-TLV is reserved for MT Management Prefix Color. No. of Octets +--------------------------------+ | MT IPv4 Prefix Color Value | 4 +--------------------------------+ The value of 1 of this sub-TLV is reserved for MT Management Prefix Color. If the MT Management Prefix color is present in this TLV, and the router announces this TLV has the Generic MT SPF enabled, the local router MAY need to install this prefix into the ``standard'' or ``management'' RIB as described in section 9.1. 11.4. Multi-Topology Reachable IPv6 Prefixes TLV TLV number of this TLV is 237. It is aligned with IPv6 Reachability TLV type 236 beside an additional two bytes in front. x CODE - 237 x LENGTH - total length of the value field x VALUE - 2-byte MT membership plus the format of IPv6 Reachability TLV, structured as follows: No. of Octets +--------------------------------+ |R |R |R |R | MT ID | 2 +--------------------------------+ | IPv6 Reachability format | 6 - 253 +--------------------------------+ . . . . +--------------------------------+ | IPv6 Reachability format | 6 - 253 +--------------------------------+ Przygienda, Shen, Sheth Expires December 2002 [Page 8] Internet Draft M-ISIS June 2002 Bits R are reserved, should be set to 0 on transmission and ignored on receipt. MT ID is a 12-bit field containing the ID of the topology being announced. After the 2-byte MT membership format, the MT IPv6 context is in the same format as IPv6 Reachability TLV, type 236 [H01]. This TLV can occur multiple times. 11.5. Reserved MT ID Values Certain MT topologies are assigned to serve pre-determined purposes: - MT ID #0: Equivalent to the ``standard'' topology. - MT ID #1: Reserved for in-band management purposes. - MT ID #2: Reserved for IPv6 routing topology. - MT ID #3: Reserved for IPv4 multicast routing topology. - MT ID #4-#3995: Reserved for IETF consensus. - MT ID #3996-#4095: Reserved for development, experimental and proprietary features. 12. Acknowledgments The authors would like to thank Andrew Partan, Dino Farinacci, Derek Yeung, Alex Zinin, Stefano Previdi, Heidi Ou, Steve Luong, and Mike Shand for the discussion, their review, comments and contributions to this draft. 13. Security Consideration ISIS security applies to the work presented. No specific security issues with the proposed solutions are known. The authentication procedure for ISIS PDUs is the same regardless of MT information inside the ISIS PDUs. 14. Normative References [ISO10589] ISO. Intermediate System to Intermediate System Routing Exchange Protocol for Use in Conjunction with the Protocol for Providing the Connectionless-Mode Network Service. ISO 10589, 1992. [RFC1195] R. Callon. Use of OSI ISIS for Routing in TCP/IP and Dual Environments. RFC 1195, December 1990. Przygienda, Shen, Sheth Expires December 2002 [Page 9] Internet Draft M-ISIS June 2002 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [LS01] T. Li and H. Smit. IS-IS Extensions for Traffic Engineering. draft-ietf-isis-traffic-04.txt, August 2001 (work in progress) [H01] C. Hopps. Routing IPv6 with IS-IS. draft-ietf-isis-ipv6-02.txt, April 2001 (work in progress) 15. Authors' Addresses Tony Przygienda Redback Networks 350 Holger Way San Jose, CA, 95134 USA prz@redback.com Naiming Shen Redback Networks 350 Holger Way San Jose, CA, 95134 USA naiming@redback.com Nischal Sheth Juniper Networks 1194 North Mathilda Avenue Sunnyvale, CA 94089 USA nsheth@juniper.net Przygienda, Shen, Sheth Expires December 2002 [Page 10]