Integrated ISIS 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.

NOTE Introduced in Cisco IOS Release 12.0(24)S, the OSPF incremental SPF feature is more efficient than the full SPF algorithm and allows OSPF to converge on a new routing topology more quickly. Information on this feature is available in OSPF Incremental SPF, at http:// products_configuration_guide_chapter09186a00804556a5.html.

■ 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.

Integrated IS-IS offers inherent support for LAN and point-to-point environments only, whereas NBMA point-to-multipoint environment support is not included. In NBMA environments, point-to-point links (subinterfaces) must be established for correct Integrated IS-IS operation.

As mentioned, one disadvantage of Integrated IS-IS is its close association with the OSI world. Because few network administrators have adequate knowledge of OSI addressing and operation, implementation of Integrated IS-IS might be difficult.

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