Chapter 2 Advanced Routing for FortiOS 5.0 : Intermediate System to Intermediate System Protocol (IS-IS) : IS-IS background and concepts : How IS-IS works : TLV
  
TLV
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.
To determine which TLVs are responsible for route-leaking, refer to “Default routing”.
To determine which TLVs are responsible for authentication, refer to “Authentication”.
To determine which TLVs are responsible for IPv4 and IPv6 awareness and reachability, refer to “Integrated IS-IS”.
For a complete list of reserved TLV codepoints, refer to RFC 3359.