IS-IS is a link-state dynamic routing protocol initially designed by the OSI. To support IPv4 routing, IS-IS is applied to IPv4 networks and called Integrated IS-IS.
As IPv6 networks are built, IS-IS also needs to provide accurate routing information for IPv6 packet forwarding. IS-IS has good scalability, supports IPv6 network layer protocols, and is capable of discovering, generating, and forwarding IPv6 routes.
Extended IS-IS for IPv6 is defined in the draft-ietf-isis-ipv6-05 of the IETF. To process and calculate IPv6 routes, IS-IS uses two new TLVs and one network layer protocol identifier (NLPID).
The two TLVs are as follows:
TLV 236 (IPv6 Reachability): describes network reachability by defining the route prefix and metric.
TLV 232 (IPv6 Interface Address): is similar to the IP Interface Address TLV of IPv4, except that it changes a 32-bit IPv4 address to a 128-bit IPv6 address.
The NLPID is an 8-bit field that identifies the protocol packets of the network layer. The NLPID of IPv6 is 142 (0x8E). If IS-IS supports IPv6, it advertises routing information through the NLPID value.