Figure 44. ISO Address FormatMulti-Topology IS-ISMulti-topology IS-IS (MT IS-IS) allows you to create multiple IS-IS topologies on a single router with separate databases. Use thisfeature to place a virtual physical topology into logical routing domains, which can each support different routing and securitypolicies.All routers on a LAN or point-to-point must have at least one common supported topology when operating in Multi-Topology IS-ISmode. If IPv4 is the common supported topology between those two routers, adjacency can be formed. All topologies must sharethe same set of L1-L2 boundaries.You must implement a wide metric-style globally on the autonomous system (AS) to run multi-topology IS-IS for IPv6 because theType, Length, Value (TLVs) used to advertise IPv6 information in link-state packets (LSPs) are defined to use only extended metrics.The multi-topology ID is shown in the first octet of the IS-IS packet. Certain MT topologies are assigned to serve predeterminedpurposes:• MT ID #0: Equivalent to the “standard” topology.• MT ID #1: Reserved for IPv4 in-band management purposes.• MT ID #2: Reserved for IPv6 routing topology.• MT ID #3: Reserved for IPv4 multicast routing topology.• MT ID #4: Reserved for IPv6 multicast routing topology.• MT ID #5: Reserved for IPv6 in-band management purposes.Transition ModeAll routers in the area or domain must use the same type of IPv6 support, either single-topology or multi-topology. A router operatingin multi-topology mode does not recognize the ability of the single-topology mode router to support IPv6 traffic, which leads to holesin the IPv6 topology.While in Transition mode, both types of TLVs (single-topology and multi-topology) are sent in LSPs for all configured IPv6 addresses,but the router continues to operate in single-topology mode (that is, the topological restrictions of the single-topology mode remainin effect). Transition mode stops after all routers in the area or domain have been upgraded to support multi-topology IPv6. After allrouters in the area or domain are operating in multi-topology IPv6 mode, the topological restrictions of single-topology mode are nolonger in effect.Interface SupportMT IS-IS is supported on physical Ethernet interfaces, physical synchronous optical network technologies (SONET) interfaces, port-channel interfaces (static and dynamic using LACP), and virtual local area network (VLAN) interfaces.AdjacenciesAdjacencies on point-to-point interfaces are formed as usual, where IS-IS routers do not implement MT extensions.If a local router does not participate in certain MTs, it does not advertise those MT IDs in its IS-IS hellos (IIHs) and so does notinclude that neighbor within its LSPs. If an MT ID is not detected in the remote side’s IIHs, the local router does not include thatIntermediate System to Intermediate System 353
