Information Technology Reference
In-Depth Information
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Routes labeled as intra-area, interarea, external Type 1, or external Type 2.
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Supports authentication.
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Uses Dijkstra algorithm to calculate SPF tree.
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Default administrative distance is 110.
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Uses multicast 224.0.0.5 (ALLSPFrouters).
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Uses multicast 224.0.0.6 (ALLDRrouters).
IS-IS
IS-IS is an International Organization for Standardization (ISO) dynamic routing specification.
IS-IS is described in ISO/IEC 10589. IS-IS is a link-state routing protocol that floods link-state
information throughout the network to build a picture of network topology. IS-IS was primarily
intended for routing OSI Connectionless Network Protocol (CNLP) packets but has the capability
to route IP packets. When routing IP packets, Integrated IS-IS provides the ability to route
protocols outside of the OSI family, such as IP.
Similar to OSPF, IS-IS establishes a hierarchical architecture of the network. IS-IS creates two
levels of hierarchy, with Level 1 (L1) for intra-area and Level 2 (L2) for interarea routing.
IS-IS distinguishes between L1 and L2 routers. (A router is called an IS in IS-IS.) L1 ISs
communicate with other L1 ISs in the same area. L2 ISs route between L1 areas and form an
intradomain routing backbone. Hierarchical routing simplifies backbone design because L1 ISs
only need to know how to get to the nearest L2 IS.
In IS-IS, a router is usually the intermediate system (IS) and personal computers, workstations,
and servers are end systems (ES).
NOTE
IS-IS Metrics
IS-IS uses a single required default metric with a maximum path value of 1023. The metric is
arbitrary and typically is assigned by a network administrator. Any single link can have a
maximum value of 64, unless the wide metric extension is implemented. IS-IS extensions
enable a three- and four-byte default metric value. Path links are calculated by summing link
values. Maximum metric values were set at these levels to provide the granularity to support
various link types, while at the same time ensuring that the SPF algorithm for route computation
is reasonably efficient. In Cisco routers, all interfaces have the default metric of 10. The
administrator must configure the interface metric if a different value is required.
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