Content Overview In recent years, the Intermediate System-to-Intermediate System (IS-IS) routing protocol has become increasingly popular, with widespread usage among service providers. IS-IS enables very fast convergence and is very scalable. It is also a very flexible protocol and has been extended to incorporate leading edge features, such as Multiprotocol Label Switching Traffic Engineering (MPLS/TE).The features of IS-IS include the following: IS-IS is a dynamic, link-state, intradomain, interior gateway protocol (IGP). It is part of the Open Systems Interconnection (OSI) suite of protocols, and an open-standard protocol based primarily on RFC 2328. The OSI suite uses Connectionless Network Service (CLNS) to provide connectionless delivery of data, and the Layer 3 protocol of Connectionless Network Protocol (CLNP). CLNP is the solution for “unreliable” (connectionless) delivery of data, similar to IP. IS-IS uses CLNS addresses to identify the routers and to build the link-state database (LSDB). The original IS-IS operates in CLNS only. However, Integrated IS-IS supports IP routing as well as CLNS. CLNS addresses are required for configuring and troubleshooting IS-IS, even when it is used only for IP. IS-IS supports different data-link environments, such as Ethernet and Frame Relay. ISIS supports variable-length subnet masking (VLSM) and rapid convergence as does Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP). Each protocol has advantages and disadvantages, but this commonality makes all of them scalable and appropriate for supporting today’s large-scale networks. This module provides an overview of the IS-IS protocol and basic configuration examples. Each of the important IS-IS commands is explained and described in an example.
Content 4.1 IS-IS Fundamentals 4.1.1 Uses for IS-IS Routing The simplicity and stability of IS-IS makes it robust in large internetworks. It is a proven and extensible IP routing protocol that converges quickly and supports VLSM. Originally developed by the Digital Equipment Corporation in the late 1980s, IS-IS was later handed over to the ISO for standardization. IS-IS was published as a public standard in ISO 9542 and republished as RFC 995. Integrated IS-IS (or dual IS-IS), which provides support for IP routing, is specified in RFC 1195 and offers support for IP and OSI protocols. Large ISPs chose IS-IS because of its scalability, convergence, and stability. One reason IS-IS is popular with ISP networks is because in 1990 the U.S. government required ISPs to support the OSI protocols, although this requirement was later dropped. IS-IS is comparable to, and in some cases preferable to, OSPF. However, it is harder to find information and expertise on IS-IS. Nevertheless, some of the largest networks in the world continue to use IS-IS, which is a tribute to its capabilities. Web Links Introduction to Intermediate System-to-Intermediate System Protocol
http://www.cisco.com/warp/public/cc/pd/iosw/prodlit/
insys_wp.pdf#search=%22AFI%20IS-IS%22 IS-IS Support Page
http://www.cisco.com/en/US/tech/tk365/tk381/
tsd_technology_support_sub-protocol_home.html
Content 4.1 IS-IS Fundamentals 4.1.2 IS-IS and OSPF IS-IS and OSPF are more similar than dissimilar. Both routing protocols have the following characteristics: The ISO developed IS-IS, and the Internet Engineering Task Force (IETF) developed OSPF. Because most of the development of these two routing protocols was done concurrently, the development groups produced two protocols that are very similar and each protocol benefited because of the other. The practical differences between the two protocols deal with perceived issues of resource usage and the ability to customize their behavior. Most debates of the merits of these protocols are colored by the fact that different groups with different cultures developed the protocols. Digital Equipment Corporation originally developed IS-IS for DECnet Phase V. In 1987, it was selected by ANSI to be the OSI IGP. At that time, it was capable of routing CLNP only. ISO is an international standards development process. According to Christian Huitema in his book Routing in the Internet, groups within the ISO and outside the U.S. did not approve of TCP/IP because of its origin (it was also called the U.S. Department of Defense protocol). From the perspective of ISO, IP development was chaotic and imprecise, based on the famous maxim of “loose consensus and running code.” From the perspective of the early Internet engineers, the ISO process was slow, irritating, and disenfranchising. In 1988, the U.S. National Science Foundation Network (NSFnet) was created. The IGP used was based on an early draft of IS-IS. The extensions to IS-IS for handling IP were developed in 1988. OSPF development began during this time and was loosely based on IS-IS. In 1989, OSPF version 1 (OSPF v1) was published, and conflict ensued between the proponents of IS-IS and OSPF. The IETF eventually supported both, although it continued to favor OSPF. With the unofficial endorsement of the IETF, OSPF eventually became the more popular protocol. By the mid-1990s, large ISPs in need of an IGP selected IS-IS for two reasons. First, IS-IS supported both CLNS and IP, which solved two problems at once. Second, OSPF was seen as immature at the time.
Content 4.1 IS-IS Fundamentals 4.1.3 IS-IS Routing ISO specifications refer to routers as intermediate systems (ISs). Therefore, IS-IS is a protocol that allows routers to communicate with other routers. The original OSI protocol suite supported numerous standard protocols at the physical, data link, network, transport, session, presentation, and application layers. Figure displays the structure of the OSI protocol suite and the OSI reference model. The OSI protocol suite uses CLNS to provide connectionless delivery of data, and the actual Layer 3 protocol is the CLNP. CLNP is the solution for "unreliable" (connectionless) delivery of data, similar to IP. IS-IS requires CLNS addresses to identify the routers and to build the LSDB. IS-IS serves as an IGP for the CLNS. Note
The original IS-IS implementation only supported CNLS. A newer version called Integrated IS-IS supports IP routing, CLNS routing, or a combination of the two. This module focuses on Integrated IS-IS with support for IP. Figure compares IP services and the equivalent OSI service. Web Links OSI Protocols
http://www.cisco.com/univercd/cc/td/doc/cisintwk/
ito_doc/osi_prot.htm#xtocid12
Content 4.1 IS-IS Fundamentals 4.1.4 IS-IS Features IS-IS is the dynamic link-state routing protocol for the OSI protocol stack. It distributes routing information for routing CLNP data for the ISO CLNS environment. The operation of IS-IS is similar to that of OSPF. IS-IS allows the routing domain to be partitioned into areas. IS-IS routers establish adjacencies using a hello protocol, and exchange link-state information using link-state packets (LSPs) throughout an area to build the LSDB. Each router then runs Dijkstra’s SPF algorithm against its LSDB to select the best paths. A minimal amount of information is communicated between areas, which reduces the