Intermediate System-to-Intermediate System (IS-IS) encapsulates its data at the Data Link layer of the Open Systems Interconnection (OSI) model and can therefore be used with both IP and Connectionless Network Protocol (CLNP). When IS-IS encapsulates packets, it uses 0xFE and 0xFEFE in the Layer 2 header to identify the Layer 3 protocol as OSI. IS-IS is specified in International Organization for Standardization (ISO) 10589.

IS-IS is a linkstate routing protocol. Routers that use a linkstate routing protocol maintain a complete topology of the network by flooding the state of each router's links across the entire network until each of the routers has information about all of the other routers in the autonomous system (AS). IS-IS uses the following Data Link layer multicast addresses to send hello packets and linkstate information:
0180.C200.0014 -All Level 1 (L1) Intermediate Systems
0180.C200.0015 -All Level 2 (L2) Intermediate Systems
0900.2B00.0005 -All Intermediate Systems
0900.2B00.0004 -All End Systems

IS-IS uses a designated intermediate system (DIS) in a broadcast multiaccess network. All IS-IS routers on the network segment establish adjacencies with the DIS. The DIS serves as a focal point for the distribution of IS-IS routing information. Once elected, the DIS must relinquish its duties if another router with a higher priority joins the network. If the DIS is no longer detected on the network, a new DIS is elected based on the priority of the remaining routers on the network segment. If a new DIS cannot be elected based solely on router priority, the highest Media Access Control (MAC) address is used. If there is still a tie, the highest system ID is the deciding factor. Every IS-IS router is required to have a unique system ID.