Network Working Group Peter Psenak Internet Draft Sina Mirtorabi Expiration Date: September 2004 Abhay Roy File name: draft-psenak-mt-ospf-00.txt Liem Nguyen Cisco Systems March 2004 MT-OSPF: Multi Topology (MT) Routing in OSPF 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "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. Abstract This draft describes the extension to OSPF in order to define independent IP topologies called Multi-Topologies (MTs). The MT extension can be used for computing different paths for different classes of service, in-band management network or incongruent topologies for unicast and multicast. M-ISIS [1] describes a similar mechanism for ISIS. 1. Introduction OSPF uses a fixed packet format, therefore it is not easy to introduce any backward compatible extensions. However the OSPF specification [2] introduced TOS metric in an earlier specification Psenak et al. [Page 1] Internet Draft MT-OSPF March 2004 [3] in order to announce a different link's cost based on TOS. The TOS based routing as described in [3] was never deployed in the field and later was removed from the spec. We propose to reuse the TOS based metric fields. They have been redefined as MT-ID and MT-ID Metric, to announce different topologies by advertising different metrics. 2. Terminology We define the following terminology in this document: Non-MT router : Routers that does not have the MT capability MT router : Routers that have MT capability as described in this document Default topology : Topology that is build using the TOS 0 metric (default metric) MT topology : Topology that is built using the corresponding MT-ID metric MT#0 topology : Representation of TOS 0 metric in MT-ID format Non-MT-Area : An area that contains only non-MT routers MT-Area : An area that contains both non-MT routers and MT routers in Compatible mode or all MT routers in Enabled mode (see section 4). 3. MT-bit in Hello packet We reuse the T-bit recalled (MT) in the option field in order to enforce that an Enabled mode router only interact with Enabled mode router. +--+--+--+--+--+--+--+--+ |DN|O |DC|EA|NP|MC|E |MT| +--+--+--+--+--+--+--+--+ MT bit: This bit MUST be set in the Hello packet only if MTRoutingCapability is Enabled, see section 4. Psenak et al. [Page 2] Internet Draft MT-OSPF March 2004 4. New parameter in the Area Data Structure We define a new parameters in the Area Data Structure: MTRoutingCapability This is a configurable parameter that will be used to facilitate the introduction of MT routers in an area and ensure the backward compatibility. This flag can take two values: Compatible or Enabled. When an area data structure is created its value is set to Compatible. If MTRoutingCapability is set to Compatible: o MT bit MUST be cleared in the Hello packet o The router MUST include all links in default topology (by using the default metric field as it is done in standard OSPF [2]) so that MT routers interact correctly with non-MT routers. If MTRoutingCapability is set to Enabled: o MT bit MUST be set in the Hello packet o The router will only accept a Hello if the MT bit is set, see section 7 5. Adjacency for MTs Each interface can be configured to belong to a set of topologies. A single adjacency will be formed with the remote neighbor even if the interface is configured to participate in multiple topologies and independently of the MT-IDs. 6. Sending OSPF control packets OSPF control packets should be sent over the default topology. If MTRoutingCapability is set to Enabled and default topology is not configured on the interface, connected route should still exist for a default topology that would enable the OSPF control packets to be received. OSPF control packets sent to the remote end-point of the virtual link may need to traverse multiple hops. These control packets should be correctly classified by the routers as packets belonging to the default topology. Event though the VL may belong to other than default topology (or multiple of them), OSPF control packets sent to the remote end of the virtual link should be forwarded using the default topology. Psenak et al. [Page 3] Internet Draft MT-OSPF March 2004 7. Forming adjacency In order to have a smooth transition from a non-MT area to MT-area, a MT router with MTRoutingcapability set to Compatible will form adjacency with non-MT routers and it will include all links as part of default topology. A link can cease participating in default topology if MTRoutingcapability is set to Enabled. In this state a router will only form adjacency with routers that set the MT bit in their hello packets, this will ensure that all routers are in Enabled mode before MT#0 can be removed from a link. Receiving OSPF Hello packets defined in section 8.2 of [2] are modified as follows: If the MTRoutingCapability of the Area Data structure is set to Enabled, the Hello packets are discarded if: o the received Hello packet does not have the MT bit set 8. Advertising MT adjacencies and corresponding IP prefixes We will reuse the TOS metric field in order to announce a topology or prefixes that belongs to a given MT. The TOS field is renamed to MT-ID in the LSAs payload (see Appendix A). 8.1 Intra-area routing When a router establishes a FULL adjacency over a link that belongs to a set of MTs, it will advertise the corresponding cost for each MT-ID. When MTRoutingCapability is set to Compatible, all links are automatically included in default topology, all advertised adjacency belonging to the default topology will use the TOS0 metric as in standard OSPF. When MTRoutingCapability is set to Enabled, MT#0 can be removed if a link does not participate in default topology. In that case the TOS0 metric is set to infinity (0xFFFF) and ignored during the MT#0 SPF computation. Psenak et al. [Page 4] Internet Draft MT-OSPF March 2004 When MTRoutingCapability is set to Enabled and a link participates in MT#0, MT-ID#0 metric is used to advertise the metric associated with the default topology. Further TOS0 metric is set to the same value as MT-ID#0 metric. However TOS 0 metric is ignored during SPF for default topology and only MT-ID#0 metric is used for SPF in default topology. Each MT has its own MT-ID metric field and when a link is not part of a given MT, the corresponding MT-ID will not appear in the LSA. The Network LSA does not contain any MT information as the DR is shared by all MTs and thus there is no change to the Network LSA. 8.2 Inter-area and external routing For Inter-area and external prefix information, the TOS metric fields renamed to MT-ID metric fields are used in order to announce prefixes in the corresponding topology. When a router originates a type 3/4/5/7 that belongs to a set of MTs, it will include the corresponding cost for each MT-ID. When MTRoutingCapability is set to Compatible, the router MUST participate in default topology and use the TOS0 metric as in standard OSPF. When MTRoutingCapability is set to Enabled: o if the prefix / router does not exist in default topology, TOS0 metric is set to infinity (0xFFFFFF). o if the prefix / router exist in default topology, TOS0 metric is used to announce a prefix / router in default topology. 9. Flushing MT information When a certain link/prefix that existed or was reachable in a certain topology is not anymore part of this topology or the reachability of the link/prefix in this topology was lost, a new version of the LSA that advertised the link/prefix must be originated. This new version of the LSA must not include any metric information representing the link/prefix in this topology. The MT metric in the Router-LSA can also be set to the maximum possible metric to enable the router to become a stub in a certain topology [4]. Psenak et al. [Page 5] Internet Draft MT-OSPF March 2004 10. Interoperability between MT capable and non-MT capable routers The default metric field is mandatory in all LSAs (even when metric value is 0). Even when the link or a prefix does not exist in the default topology, a non MT capable router can consider the zero value in the metric field as a valid metric and consider the link/prefix as part of the default topology. In order to prevent the above problem, a MT capable router in Compatible mode will always include all links as part of the default topology. If links need to be removed from the default topology, a MT capable router MUST be configured in Enabled mode. In this mode the router will make sure that all routers are MT Enabled in the area before forming any adjacency so that default metric field can be safely ignored during the MT#0 SPF computation. 11. MT area boundary Each OSPF interface belongs to a single area and all MTs sharing that link need to belong to the same area. Therefore the area boundaries for all MTs are the same but each MT's attachment to the area is independent. 12. MT SPF Computation By considering MT-ID metrics in the LSAs, OSPF will be able to compute multiple topologies, one for each MT the router is part of and find paths to IP prefixes for each MT independently. A separate SPF will be computed for each MT-ID to find independent paths to IP prefixes. Each nexthop computed during the MT SPF MUST belong to the same MT. Network LSAs are used by all topologies during the SPF computation. During SPF for a given MT-ID, only the link/metric for the given MT-ID will be considered. Entries in the Router Routing table will be MT-ID specific. During the SPF computation for default topology: o If MTRoutingCapability is set to Compatible, TOS0 metric is considered during the SPF computation. o If MTRoutingCapability is set to Enabled, MT-ID#0 metric in Router LSAs is considered during calculation of the shortest-path tree for an area and during intra-area route calculation. TOS0 metric is considered in LSAs Type-3/4/5/7 during inter-area and external route calculation. Psenak et al. [Page 6] Internet Draft MT-OSPF March 2004 13. MT ID Values Only MT-IDs in the range [0-127] are valid, because external LSAs use one bit in the MT-ID field (E bit) for the external metric-type. MT-ID #0 is reserved for routers in Enabled mode to advertise the default topology. Any invalid MT-ID should be ignored. 14. Migration from non-MT-Area to MT-area Introducing MT-OSPF in a network can be gradually done since MT routers in Compatible mode will interact in default topology with non-MT routers yet exchanging MT topologies with other MT capable routers. If there is a requirement to exclude some links from default topology in an area, all routers MUST be in Enabled mode. In this section we describe migrations steps to consider while transitioning from a non-MT network to a MT network. Migration Steps --------------- Consider a network with a backbone area and a sets of non-backbone areas functioning in standard OSPF mode. We would like to migrate to a MT network either partially or completely. 1) Part of an area is upgraded as needed to have MT capability, the MT routers will be in Compatible mode and interact with non-MT routers in default topology, further MT routers will participate in MT topology as needed. 2) If a new non-backbone area is created for MT routers, it may be set to Enabled mode as there is no interaction required with non-MT routers, in this mode default topology can be excluded if required. 3) If there is more than one non-backbone areas where MT is being used, it is desirable that area 0 be first upgraded to MT capable routers so that inter-area routing is assured for MT destinations in different areas. 4) Gradually the whole network can be made MT aware Note that Inter-area routing for the MT-area still depends on the backbone area. Therefore if different areas in a given MT-ID need to communicate, the backbone area also needs to be configured for this MT-ID. Psenak et al. [Page 7] Internet Draft MT-OSPF March 2004 15. Forwarding in MT Forwarding must make sure that only routes belonging to the single topology are used to forward the packet along its way from source to destination, therefore user configuration MUST be consistently applied throughout the network so that an incoming packet is associated with the same topology on each hop as it is being forwarded. It is outside of the scope of this document to consider different methods of associating an incoming packet to the corresponding MT RIB. 16. Acknowledgments The authors would like to thank Scott Sturgess and Alvaro Retana for their comments on the document. 17. Security Consideration No specific security issues with the proposed solutions are known. 18. References [1] Przygienda, Shen, Sheth, "Multi Topology (MT) Routing in IS-IS", draft-ietf-isis-wg-multi-topology-06.txt, Work in progress. [2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. [3] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994. [4] Retana, Nguyen, White, "OSPF Stub Router Advertisement", RFC 3137, June 2001. Appendix A. LSAs content defined in [2] are modified to introduce MT-ID. A.1 Router-LSAs Router-LSAs are the Type 1 LSAs. Each router in an area originates a router-LSA. The LSA describes the state and cost of the router's links (i.e., interfaces) to the area. All of the router's links to the area must be described in a single router-LSA. For details concerning the construction of router-LSAs, see Section 12.4.1. [2] Psenak et al. [Page 8] Internet Draft MT-OSPF March 2004 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |*|*|*|N|W|V|E|B| 0 | # links | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | # MT-ID | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MT-ID | 0 | MT-ID metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MT-ID | 0 | MT-ID metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | A.2 Network-LSAs Network-LSAs are the Type 2 LSAs. A network-LSA is originated for each broadcast and NBMA network in the area which supports two or more routers. The network-LSA is originated by the network's Designated Router. The LSA describes all routers attached to the network, including the Designated Router itself. The LSA's Link State ID field lists the IP interface address of the Designated Router. Psenak et al. [Page 9] Internet Draft MT-OSPF March 2004 The distance from the network to all attached routers is zero. This is why metric fields need not be specified in the network-LSA. For details concerning the construction of network-LSAs, see Section 12.4.2. [2] 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attached Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Note that network LSA does not contain any MT-ID field as the cost of the network to the attached routers is 0 and DR is shared by all MT. A.3 Summary-LSAs Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated by area border routers. Summary-LSAs describe inter-area destinations. For details concerning the construction of summary- LSAs, see Section 12.4.3. [2] Type 3 summary-LSAs are used when the destination is an IP network. In this case the LSA's Link State ID field is an IP network number (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix E [2] for details). When the destination is an AS boundary router, a Type 4 summary-LSA is used, and the Link State ID field is the AS boundary router's OSPF Router ID. (To see why it is necessary to advertise the location of each ASBR, consult Section 16.4 of [2]). Other than the difference in the Link State ID field, the format of Type 3 and 4 summary-LSAs is identical. Psenak et al. [Page 10] Internet Draft MT-OSPF March 2004 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 3 or 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MT-ID | MT-ID metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MT-ID | MT-ID metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A.4.5 AS-external-LSAs AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by AS boundary routers, and describe destinations external to the AS. For details concerning the construction of AS-external-LSAs, see Section 12.4.3. [2] AS-external-LSAs usually describe a particular external destination. For these LSAs the Link State ID field specifies an IP network number (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix E [2] for details). AS- external-LSAs are also used to describe a default route. Default routes are used when no specific route exists to the destination. When describing a default route, the Link State ID is always set to DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0. Psenak et al. [Page 11] Internet Draft MT-OSPF March 2004 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS age | Options | 5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS checksum | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| 0 | metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Forwarding address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | External Route Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| MT-ID | MT-ID metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Forwarding address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | External Route Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| MT-ID | MT-ID metric | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Forwarding address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | External Route Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Authors' address Peter Psenak Abhay Roy Cisco Systems Cisco systems Parc Pegasus, 170 W. Tasman Dr. De Kleetlaan 6A San Jose, CA 95134 1831 Diegem, Belgium USA E-mail: ppsenak@cisco.com E-mail: akr@cisco.com Sina Mirtorabi Liem Nguyen Cisco Systems Cisco Systems 225 West Tasman drive 7025 Kit Creek Rd. San Jose, CA 95134 Research Triangle Park, NC 27709 USA USA E-mail: sina@cisco.com E-mail: lhnguyen@cisco.com Psenak et al. [Page 12]