• area address — within your routing domain or area, each area must have a unique area value. The first byte is called the authority andformat indicator (AFI).• system address — the router’s MAC address.• N-selector — this is always 0.The following illustration is an example of the ISO-style address to show the address format IS-IS uses. In this example, the first five bytes(47.0005.0001) are the area address. The system portion is 000c.000a.4321 and the last byte is always 0.Figure 58. 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 this feature toplace a virtual physical topology into logical routing domains, which can each support different routing and security policies.All routers on a LAN or point-to-point must have at least one common supported topology when operating in Multi-Topology IS-IS mode. IfIPv4 is the common supported topology between those two routers, adjacency can be formed. All topologies must share the same set ofL1-L2 boundaries.You must implement a wide metric-style globally on the autonomous system (AS) to run multi-topology IS-IS for 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 to serve 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 inmulti-topology mode does not recognize the ability of the single-topology mode router to support IPv6 traffic, which leads to holes in theIPv6 topology.While in Transition mode, both types of TLVs (single-topology and multi-topology) are sent in LSPs for all configured IPv6 addresses, butthe router continues to operate in single-topology mode (that is, the topological restrictions of the single-topology mode remain in effect).Transition mode stops after all routers in the area or domain have been upgraded to support multi-topology IPv6. After all routers in the areaor domain are operating in multi-topology IPv6 mode, the topological restrictions of single-topology mode are no longer in effect.Intermediate System to Intermediate System 441
