IS-IS was developed by Digital Equipment Corporation (DEC) as the dynamic link-state routing protocol for the Open Systems Interconnection (OSI) protocol suite. The OSI suite uses Connectionless Network Service (CLNS) to provide connectionless delivery of data, and the actual Layer 3 protocol is Connectionless Network Protocol (CLNP). CLNP is the OSI suite solution for connectionless delivery of data, similar to IP in the TCP/IP suite. IS-IS uses CLNS addresses to identify the routers and build the LSDB.

Even if Integrated IS-IS is used only for routing IP (and not CLNP), OSI protocols are used to form the neighbor relationships between the routers; therefore, for Integrated ISIS to work, CLNS addresses must still be assigned to areas. This proves to be a major disadvantage when implementing Integrated IS-IS, because OSI and CLNS knowledge is not widespread in the enterprise networking community. Therefore, Integrated IS-IS is not recommended as an enterprise routing protocol; it is included here for completeness.

Integrated IS-IS Characteristics

IS-IS is a popular IP routing protocol in the ISP industry. The simplicity and stability of IS-IS make it robust in large internetworks. Integrated IS-IS characteristics include the following:

■ VLSM support: As a classless routing protocol, Integrated IS-IS supports VLSM.

■ Fast convergence: Similar to OSPF, Integrated IS-IS owes its fast convergence characteristics to its link-state operation (including flooding of triggered link-state updates). Another feature that guarantees fast convergence and less CPU usage is the partial route calculation (PRC). Although Integrated IS-IS uses the same algorithm as OSPF for best path calculation, the full SPF calculation is initially performed on network startup only. When IP subnet information changes, only a PRC for the subnet in question runs on routers. This saves router resources and enables faster calculation. A full SPF calculation must be run for each OSPF change.

■ Excellent scalability: Integrated IS-IS is more scalable and flexible than OSPF; IS-IS backbone area design is not as strict as OSPF, thereby allowing for easy backbone extension.

■ Reduced bandwidth usage: Triggered updates and the absence of periodic updates ensure that less bandwidth is used for routing information.

Table summarizes some characteristics of IP routing protocols discussed in this chapter. Although they are no longer recommended enterprise protocols, RIPv1, RIPv2, and IGRP are also included in this table for completeness.

■ Very good scalability: OSPF’s multiple area structure provides good scalability. However, OSPF’s strict area implementation rules require proper design to support other scalability features such as manual summarization of routes on ABRs and ASBRs, stub areas, totally stubby areas, and not-so-stubby areas (NSSA). The stub, totally stubby, and NSSA features for nonbackbone areas decrease the amount of LSA traffic from the backbone (area 0) into nonbackbone areas (and they are described further in the following sidebar). This allows lowend routers to run in the network’s peripheral areas, because fewer LSAs mean smaller OSPF topology tables, less OSPF memory usage, and lower CPU usage in stub area routers.

■ Reduced bandwidth usage: Along with the area structure, the use of triggered (not periodic) updates and manual summarization reduces the bandwidth used by OSPF by limiting the volume of link-state update propagation. Recall, though, that OSPF does send updates every 30 minutes.

■ VLSM support: Because OSPF is a classless routing protocol, it supports VLSM to achieve better use of IP address space.