The Intermediate System-to-Intermediate System (IS-IS) and Open Shortest Path First (OSPF) routing protocols are similar in that they both flood routing information to every router in the autonomous system (AS) and they can both perform address summarization. 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 AS.

ISIS and OSPF can both perform address summarization. Summarization helps to reduce the size of the routing table. You can issue the summary-address command to configure IS-IS to summarize routes. You can issue the area range command on an Area Border Router (ABR) to configure OSPF to summarize internal routes at the area boundary.

Although IS-IS does not have a backbone area, ISIS requires that all Level 2 (L2) and Level 1/Level 2 (L1/L2) routers be connected to form a backbone through the routing domain. OSPF has a backbone area that is used for interarea routing. With OSPF, all routers that connect to multiple areas are required to attach to the backbone area.

ISIS does not have a designated router (DR) or a backup designated router (BDR). IS-IS uses a designated intermediate system (DIS) in a broadcast multiaccess network. The DIS is analogous to the OSPF DR. All IS-IS routers on the network segment establish adjacencies with the DIS. The DIS serves as a focal point for the distribution of ISIS 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.

OSPF requires the election of a DR and a BDR in a multiaccess network. All OSPF routers on the multiaccess network segment are required to establish adjacencies with only the DR and the BDR. The DR and BDR serve as a focal point for the distribution of OSPF routing information. The BDR is elected first, and the DR is elected afterward. Once elected, the DR does not relinquish its duties if another more suitable router joins the network. If the DR is no longer detected on the network, the BDR immediately assumes the role of the DR and a new BDR is elected. Both ISIS and OSPF are standards, but ISIS is not an Internet Engineering Task Force (IETF) standard? it is an Open Systems Interconnection (OSI) standard. ISIS is specified in International Organization for Standardization (ISO) 10589. By contrast, OSPF is an IETF standard. OSPFv2 is specified in Request for Comments (RFC) 2328, and OSPFv3 is specified in RFC 5340.

IS-IS can route IP and Connectionless Network Protocol (CLNP). IS-IS encapsulates its data directly at the Data Link layer and is therefore not dependent on the Network layer protocol. IS-IS uses Data Link layer multicast addresses to send hello packets and linkstate information. OSPF was designed to work in conjunction only with IP? OSPFv2 can route only IPv4, and OSPFv3 can route only IPv4 and IPv6. OSPF encapsulates its data at the Network layer, making it dependent on the supporting Network layer protocol. OSPF uses Network layer multicast addresses to send hello packets and linkstate information.