You must implement a wide metric-style globally on the autonomous system (AS) to run multi-topology IS-ISfor IPv6 because the Type, 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 toserve predetermined purposes:• 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 operating in multi-topology mode does not recognize the ability of the single-topologymode router to support IPv6 traffic, which leads to holes in the IPv6 topology.While in Transition mode, both types of TLVs (single-topology and multi-topology) are sent in LSPs for allconfigured IPv6 addresses, but the router continues to operate in single-topology mode (that is, thetopological restrictions of the single-topology mode remain in effect). Transition mode stops after all routersin the area or domain have been upgraded to support multi-topology IPv6. After all routers in the area ordomain are operating in multi-topology IPv6 mode, the topological restrictions of single-topology mode areno longer 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 MTextensions.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 not include that neighbor within its LSPs. If an MT ID is not detected in the remote side’s IIHs, thelocal router does not include that neighbor within its LSPs. The local router does not form an adjacency ifboth routers do not have at least one common MT over the interface.Intermediate System to Intermediate System 584
