IS-IS uses
type-length-variable (TLV) parameters to carry information in Link-State PDUs (LSPs). Each IS-IS LSP consists of a variable-length header to which TLVs are appended in order to extend IS-IS for IP routing. The TLV field consists of one octet of type (T), one octet of length (L), and “L” octets of Value (V). They are included in all of the IS-IS
Packet types. For a complete breakdown of the LSP, refer to
“LSP structure”.
In IS-IS, TLVs are used to determine route-leaking and authentication, and are also used for IPv4 and IPv6 awareness and reachability.