tities in an IS-IS domain must be associated with a single physi- cal area, which is determined by the area ID in the router address. IS-IS packets can be divided into three categories: 1. Hello packets are used to establish and maintain adjacencies between IS-IS neighbors. 2. Link-state packets are used to distribute routing information between IS-IS nodes. 3. Sequence number packets are used to control distribution of link-state packets, essentially providing mechanisms for synchronization of the distributed link- state databases on the routers in an IS-IS routing area. Each type of IS-IS packet is made up of a header and a number of optional variable-length fields organized in records and containing specific routing-related information. The vari- able length fields are called TLV as in the case of the OSPF-TE from the form of the record that is organized into three fields: type, length, and value. Each variable-length field has a 1-byte type label (the Type field) that describes the infor- mation it contains. The second field is a length information that contains the length of the third field whose content is the specific information that is carried by the TLV (see Table 8.1 for examples). The different types of IS-IS packets have a slightly different composition of the header, but the first 8 bytes are repeated in all packets. Each type of packet then has its own set of additional header fields, which are followed by TLVs. Table 8.2 lists TLVs specified in ISO 10589 while Table 8.3 lists the TLVs introduced by IETF in RFC 1195. Enhancements to the original IS-IS protocol are normally achieved through the intro- duction of new TLV fields. Note that a key strength of the IS-IS protocol design lies in the ease of extension through the introduction of new TLVs rather than new packet types. The routing layer functions provided by the IS-IS protocol can be grouped into two main categories: subnetwork-dependent functions and subnetwork-independent functions. TABLE 8.2 List of TLVs Specified in ISO 10589 Description 