IPv6 IS-IS extends the address families supported by IS-IS to include IPv6, in addition to OSI and IPv4.
IPv6 IS-IS in IOS supports either single topology mode or multiple topology mode.
Single topology support for IPv6 allows IS-IS for IPv6 to be configured on interfaces along with other network protocols (for example, IPv4 and Connectionless Network Service). All interfaces must be configured with the identical set of network address families. In addition, all routers in the IS-IS area (for Level 1 routing) or the domain (for Level 2 routing) must support the identical set of network layer address families on all interfaces.
When single topology support for IPv6 is being used, either old or new style TLVs may be used. However, the TLVs used to advertise reachability to IPv6 prefixes use extended metrics. Cisco routers do not allow an interface metric to be set to a value greater than 63 if the configuration is not set to support only new style TLVs for IPv4. In single topology IPv6 mode, the configured metric is always the same for both IPv4 and IPv6.
IS-IS multitopology support for IPv6 allows IS-IS to maintain a set of independent topologies within a single area or domain. This mode removes the restriction that all interfaces on which IS-IS is configured must support the identical set of network address families. It also removes the restriction that all routers in the IS-IS area (for Level 1 routing) or domain (for Level 2 routing) must support the identical set of network layer address families. Because multiple SPFs are performed, one for each configured topology, it is sufficient that connectivity exists among a subset of the routers in the area or domain for a given network address family to be routable.
You can use the isis ipv6 metric command to configure different metrics on an interface for IPv6 and IPv4.
When multitopology support for IPv6 is used, enter the metric-style wide command to configure IS-IS to use new style TLVs because TLVs used to advertise IPv6 information in LSPs are defined to use only extended metrics.