Network Working Group Peter Psenak Internet Draft Sina Mirtorabi Expiration Date: April 2005 Abhay Roy File name: draft-ietf-ospf-mt-00.txt Liem Nguyen Cisco Systems Padma Pillay-Esnault Juniper Networks October 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 describes a similar mechanism for ISIS. This draft also describes an optional extension of Multi-topologies whereby some links might be excluded from the default topology. Psenak et al. [Page 1] Internet Draft MT-OSPF October 2004 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 [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 separate metrics for each of them. 2. Terminology We define the following terminology in this document: Non-MT router : Routers that do not have the MT capability MT router : Routers that have MT capability as described in this document MT-ID : Renamed TOS field in LSAs to represent multi topology ID. Default topology : Topology that is built 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 or only MT routers 3. 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. 4. 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. Psenak et al. [Page 2] Internet Draft MT-OSPF October 2004 5. Sending OSPF control packets OSPF control packets should be sent over the default topology. 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. 6. 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). 6.1 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. All links are by default included in default topology, all advertised adjacency belonging to the default topology will use the TOS0 metric as in standard OSPF. Each MT has its own MT-ID metric field and when a link is not part of a given MT, the corresponding MT-ID metric 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. 6.2 Inter-area and external routing In Summary and External LSAs, the TOS metric fields are renamed to MT-ID metric fields and are used in order to announce prefix/router reachability in the corresponding topology. When a router originates a type 3/4/5/7 LSA that belongs to a set of MTs, it will include the corresponding cost for each MT-ID. The router by default participate in default topology and use the TOS0 metric for default topology as in standard OSPF. Psenak et al. [Page 3] Internet Draft MT-OSPF October 2004 7. Flushing MT information When a certain link/prefix that existed or was reachable in a certain topology is no longer 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]. 8. 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 the TOS0 metric is considered during the SPF computation. 9. 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. Following MT-ID values are reserved: 0 - reserved for routers in MTRoutingExclusionCapability mode to advertise the metric associated with default topology (see section 11.2). 1 - reserved for default multicast topology. Any unknown MT-ID should be ignored. 10. 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. Psenak et al. [Page 4] Internet Draft MT-OSPF October 2004 11. Exclusion of links in the default topology The multi-topologies imply that all the routers participate in the default topology. However, it is useful in some circumstances to exclude some links from the default topology and reserve them for some specific classes of traffic. The multi-topologies extension for default topology link exclusion is described in the following sections. 11.1 MT-bit in Hello packet OSPF does not have the notion of unreachable link. All links can have a maximum metric of 0xFFFF carried in the Router LSA. The link exclusion capability requires routers to ignore TOS 0 metric in router-LSA in default topology and use MT-ID#0 metric instead to advertise the metric associated with the default topology. Hence, all routers within an area MUST agree on how the metric for default topology will be advertised. The unused T-bit is renamed (MT) in the option field in order to enforce that a multi-topology link-excluding capable router will only interact with another similarly configured router. +---+---+---+---+---+---+---+---+ |DN |O |DC |EA |NP |MC |E |MT | +---+---+---+---+---+---+---+---+ MT-bit: This bit MUST be set in the Hello packet only if MTRoutingExclusionCapability is Enabled (see section 11.2). 11.2 New parameter in the Area Data Structure We define a new parameters in the Area Data Structure: MTRoutingExclusionCapability This is a configurable parameter that will be used to facilitate the introduction of MT routers in an area and ensure the backward compatibility. By default, when an area data structure is created the MTRoutingExclusionCapabilty is disabled. If MTRoutingExclusionCapability is disabled: o MT-bit MUST be cleared in the Hello packet o If a link participates in a non-default topology, it is automatically included 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. Psenak et al. [Page 5] Internet Draft MT-OSPF October 2004 If MTRoutingExclusionCapability 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 11.3) We call MTRoutingExclusionCapability "mode", when MTRoutingExclusionCapability is set to Enabled. 11.3. Forming adjacency with link exclusion capability. In order to have a smooth transition from a non-MT area to MT-area, a MT router with MTRoutingExclusionCapability set to disable 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 MTRoutingExclusionCapability 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 default topology can be disabled on a link. Receiving OSPF Hello packets defined in section 8.2 of [2] are modified as follows: If the MTRoutingExclusionCapability 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 11.4. Sending OSPF control packets over an excluded link. If MTRoutingExclusionCapability 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 sent and received. 11.5. Modified MT SPF Computation with link exclusion capability. When MTRoutingExclusionCapability 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. When MTRoutingExclusionCapability is set to Enabled and a link participates in default topology, MT-ID#0 metric is used to advertise 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. Psenak et al. [Page 6] Internet Draft MT-OSPF October 2004 When originating Summary and External LSAs, if MTRoutingExclusionCapability 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. During the Summary and External prefix calculation for default topology TOS0 metric is used in LSA Type-3/4/5/7. 12. 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 will by default 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 MTRoutingExclusionCapability mode. In this mode the router will make sure that all routers in the area are in the MTRoutingExclusionCapability mode before forming any adjacency so that TOS0 metric field can be safely ignored during the MT#0 SPF computation. Note that for any prefix or router to become reachable in a certain topology, a contiguous path inside that topology must exist between the calculating router and the destination prefix or router. 13. Migration from non-MT-Area to MT-area Introducing MT-OSPF in a network can be gradually done since MT routers will interact in default topology with non-MT routers, yet exchanging information about other 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 MTRoutingExclusionCapability 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. Psenak et al. [Page 7] Internet Draft MT-OSPF October 2004 1) Part of an area is upgraded as needed to have MT capability, the MT routers will 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 in MTRoutingExclusionCapability 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. 14. Acknowledgments The authors would like to thank Scott Sturgess and Alvaro Retana for their comments on the document. 15. Security Consideration No specific security issues with the proposed solutions are known. 16. IANA Considerations T-bit defined in [3] for router's TOS capability is reclaimed as MT-bit in this document. Likewise, TOS field in type 1,3,4,5,7 LSAs defined in [2] is reclaimed as MT-ID in this document. 17. 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. Psenak et al. [Page 8] Internet Draft MT-OSPF October 2004 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] 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Psenak et al. [Page 9] Internet Draft MT-OSPF October 2004 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. 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. Psenak et al. [Page 10] Internet Draft MT-OSPF October 2004 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. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Psenak et al. [Page 11] Internet Draft MT-OSPF October 2004 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. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Psenak et al. [Page 11] Internet Draft MT-OSPF October 2004 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 Padma PIllay-Esnault Juniper Networks 1194 N. Mathilda Avenue Sunnyvale, CA 94089 USA E-mail: padma@juniper.net Psenak et al. [Page 12]