Standard protocols released by the IETF defines two new TLVs that can support IPv6 routes and a new Network Layer Protocol Identifier (NLPID), which ensures that IS-IS can process and calculate IPv6 routes.
The two new TLVs are as follows:
IPv6 Reachability
The IPv6 Reachability TLV indicates the reachability of a network by specifying the route prefix and metric. The type value is 236 (0xEC).
IPv6 Interface Address
The IPv6 Interface Address TLV is similar to the IP interface address TLV of IPv4 in function, except that it changes the original 32-bit IPv4 address to a 128-bit IPv6 address. The type value is 232 (0xE8).
The NLPID is an 8-bit field that identifies network layer protocol packets. The NLPID of IPv6 is 142 (0x8E). If an IS-IS router supports IPv6, it advertises routing information through the NLPID value.