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In this chapter, you learned the basics of the OSI protocols and the IS-IS and Integrated IS-IS routing protocols. You also learned how to configure and troubleshoot Integrated IS-IS for IP on a Cisco router.

An IS is a router. A domain is any portion of an OSI network that is under a common administrative authority. Within any OSI domain, one or more areas can be defined. An area is a logical entity; it is formed by a set of contiguous routers and the data links that connect them. All routers in the same area exchange information about all the hosts that they can reach. The areas are connected to form a backbone. All routers on the backbone know how to reach all areas.

IS-IS is the dynamic link-state routing protocol for the OSI protocol stack. As such, it distributes routing information for routing CLNP data for the ISO CLNS environment.

Integrated IS-IS is an implementation of the IS-IS protocol for routing multiple network protocols; it is an extended version of IS-IS for mixed ISO CLNS and IP environments, or for IP only.

OSI network layer addressing is implemented with NSAP addresses that identify any system in the OSI network. If the NSEL field of the NSAP is 00, the NSAP refers to the device itself—that is, it is the equivalent of the Layer 3 OSI address of that device. This address with the NSEL set to 00 is known as the NET. The NET is used by routers to identify themselves in the LSPs and, therefore, forms the basis for the OSI routing calculation. (The NET is a similar concept to the router identifier used by OSPF.)

Every IS-IS router requires an OSI address even if it is routing only IP. IS-IS uses the OSI address in the LSPs to identify the router, build the topology table, and build the underlying IS-IS routing tree.

It is important to note that IP information takes no part in the calculation of the SPF tree—it is simply information about leaf connections to the tree.

Troubleshooting Integrated IS-IS, even in an IP-only world, requires some investigation of CLNS data. For example, the IS-IS neighbor relationships are established over OSI, not over IP.

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