IS-IS was originally designed for IPv4, but extensions via TLVs (Type-Length-Value) allow it to support IPv6. These are referred to as New IS-IS TLVs, including TLV 236 (IPv6 Reachability), TLV 232 (IPv6 Interface Address), etc. These TLVs are crucial for IPv6 route computation within IS-IS.

Exact Extract - HCIE-Datacom Guide – IPv6 Routing Protocols Chapter

“IS-IS uses new TLVs such as TLV 236 and TLV 232 to carry IPv6 prefix and interface address information. These TLVs are essential to enable IPv6 route calculation.”