Report 6 Weijian Chai
Configuring IS-IS Protocol
MAR 21, 2003 by Catherine Paquet, Diane Teare
Article is provided courtesy of Cisco Press
Overview of IS-IS Routing
IS-IS—IS-IS is a dynamic link-state routing protocol used in an ISO CLNS environment for routing CLNP. Routers usually operate as ISs and can exchange reachability information with other ISs using the IS-IS protocol. As an IS, a Cisco router can be a Level 1 router, a Level 2 router, or a Level 1–2 router. In the latter case, the router can advertise itself at Level 1 as an exit point from the area. Integrated IS-IS allows the IS-IS protocol to propagate routing information for other protocols as well as—or instead of—CLNS. Specifically, IS-IS can route CLNS, IP, or both (this latter is called dual mode).
Who Uses IS-IS?
IS-IS is popular among telcos and large ISPs. This popularity finds its roots with ISPs that were around at the beginning of the Internet and chose IS-IS over OSPF for their IGP. It is believe that at that time, IS-IS had fewer technical limitations than OSPF as an IGP. Those ISPs have since become today's tier 1 carriers, so any appliance targeting tier 1 carriers must offer IS-IS.
The OSI protocol suite supports numerous standard protocols at the physical, data link, network, transport, session, presentation, and application layers.
OSI network layer addressing is implemented by using two types of hierarchical addresses: network service access point (NSAP) addresses and a specific subset of NSAPs cal- led network-entity titles (NETs). An NSAP is a conceptual point on the boundary between the network and transport layers. The NSAP is the location at which OSI network services are provided to the transport layer. Each transport layer entity is assigned a single NSAP, which is individually addressed in an OSI internetwork using NSAP addresses.
The OSI protocol suite specifies two routing protocols at the network layer: End System-to-Intermediate System (ES-IS) and IS-IS. In addition, the OSI suite implements two types of network services: connectionless service and connection-oriented service.
Integrated IS-IS
As previously mentioned, 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. Integrated IS-IS tags CLNP routes with information regarding IP networks and subnets. It can be used purely for IP routing, purely for ISO routing, or for a combination of the two.
Integrated IS-IS provides an alternative to OSPF in an IP environment, mixing ISO CLNS and IP routing in one protocol.
IS-IS has level 1, level 2, and level 1–2 routers.