A Brief Historical Tour

A brief historical review of each protocol might be appropriate here to set the stage for the following discussions. Both Integrated IS-IS and OSPF were specified in the latter part of the 1980s. Around 1988, the United States National Science Foundation Network (NSFnet) deployed an interior gateway protocol based on an early IS-IS draft. Around the same time, development work on OSPF was started. OSPF actually evolved from this early version of IS-IS but with IP as its basic premise . OSPF core concepts, such as the flooding of link-state information, the shortest path first algorithm (SPF) for calculating best paths, use of designated router on broadcast links, and so on were all borrowed from this earlier version of IS-IS. Version 1 of the OSPF protocol was published as RFC 1131 in October 1989. Near the same time (December 1990), Integrated IS-IS featuring extensions for IP routing was also published as RFC 1195. The IS-IS protocol was originally specifed in ISO 10589 for dynamic routing of CLNP datagrams in the ISO CLNS environment.

Cisco's implementation of Integrated IS-IS was first released in the 1991/1992 time frame. Significant enhancements to the Cisco IOS implementation of IS-IS were introduced in 1994 in conjunction with support for Netware Link Service Protocol (NLSP), which was designed for routing Novell's IPX Protocol. These enhancements improved resilience and robustness of the Cisco implementation and introduced new capabilites that attracted the interest of network architects in the Internet community looking at migrating their networks from the Routing Information Protocol (RIP) to a more robust IGP. Large scale deployment of the IS-IS protocol in major ISPs began in 1995. Another issue that potentionally played into this was the U.S. government interest in the ISO CLNS suite, which was reflected in a requirement for CLNP routing support in the NSFNet project by the National Science Foundation (NSF).

With its dual routing capabilities, IS-IS seemed to be the best choice of IGP to support integrated routing of both IP and CLNP. With a fast growing deployed base, the Cisco implementation quickly matured, becoming stable and robust for use in large networks. It is interesting to note that most of the largest ISP networks today run IS-IS for routing IP only. IS-IS still remains popular for ISO routing applications in telecommunication management networks.

Unlike IS-IS, which started life as an ISO protocol, OSPF was inherently designed to support only IP and was promoted in the IETF as the preferred IGP for IP networks ”or at least it appeared so. Because IS-IS support was not available on many routers (noticeably Bay and 3Com routers), OSPF automatically became the routing glue for reasonably large networks with multivendor router platforms. An active Working Group in the IETF and evolving specifications also went a long way to help promote OSPF; and so OSPF became more popular and more widely adopted compared to IS-IS.

Interest in Integrated IS-IS for IP routing continued to grow, and most ISPs that sprung up in Europe elected to deploy the ISO standards based on IS-IS instead of OSPF, which is an IETF standard. Some U.S.-based ISPs even migrated from OSPF to Integrated IS-IS. In the later part of the 90s, many new ISPs adopted the business models, network architecture, and protocols of the established large providers and thus went with IS-IS for IGP, further increasing the deployed base. OSPF also continued to flourish and continued to be adopted by many ISPs.

In summary, both IS-IS and OSPF have prevailed through the test of time and have established themselves as the IGPs of choice for service provider netoworks. New extensions to both protocols, such as MPLS traffic engineering, have been developed over the past two years , and with active working groups for either protocol in the IETF, they continue to evolve , essentially in lock-step fashion. OSPF has been updated by many RFCs since RFC1131, including a major revision. Version 2 of the OSPF protocol was first published in July 1991 as RFC1247. The most current IEFT standard for version 2 of the OSPF protocol was published as RFC 2328 in April 1998. All the OSPF RFCs mentioned so far address IPv4. RFC 2740 standardizes OSPF for IPv6.

Most of the original work on the OSPF protocol was done and documented by John Moy, who was then at Proteon, Inc. Moy was chairman of IETF OSPF Working Group for many years. Obviously, the many dedicated participants in the OSPF working group meetings also contributed to the shaping of the protocol in diverse ways.

IS-IS was seemingly abandoned in the IETF after the release of RFC1195 in 1990. Also, there was no major standardization effort in the ITU for a while, so ISO 10589 and RFC1195 remain the authoritative complete standards for IS-IS version 1 and Integrated IS-IS, respectively. However, the IETF IS-IS working group has been re-opened over the last couple of years to standardize various new appliciations of IS-IS, such as MPLS Traffic Engineering, IPv6, and many others. Also, a second version of the IS-IS protocol is currently being worked on with plans to capture most of the enhancements over the years in a single place. It should be noted that the memberships of both relevant ITU and IETF working groups are contributing to this effort.

This section closes with an interesting observation that OSPF seems to be more documented than IS-IS, and a large collection of technical textbooks are readily available in bookstores. There is also a larger collection of OSPF design guides, white papers, and application notes at various vendor web sites, including Cisco Connection Online (cco.cisco.com or www.cisco.com). This obvious disparity in coverage between IS-IS and OSPF is intended to be addressed by this book.