Genesis ..

Both Integrated IS-IS and OSPF were specified in the latter part of the 1980s

In 1987 OSI adopted DECnet Phase V’s routing algorithm with some modifications and named it IS-IS. Around 1988, the NSFnet deployed an IGP loosely based on an early draft of IS-IS. Around the same time, development on OSPF started which took most of the basic concepts from this early version of IS-IS but was designed to support only IPv4. In October 1989 the version 1 of OSPF was released as RFC 1131 and around the same time in December 1990, Integrated IS-IS was released and published as RFC 1195.

Version 2 of OSPF was first published in July 1991 as RFC 1247 and CISCO started shipping it. It released its implementation for Dual IS-IS in 1992. Till now numerous ISPs had deployed OSPF and very few IS-IS. In 1994 there were significant improvements done to CISCO’s IOS implementation for in conjunction with support for Network Link Service Protocol (Novell’s IPX protocol).

These enhancements improved the performance, resilience and robustness of CISCO’s implementation which made a lot of ISPs to shift to IS-IS.

By 1995 most of the major ISPs had started deploying IS-IS. What helped this further was US government’s interest in ISO CLNS suite, which was reflected in a requirement for CLNP routing support in the NSFnet project by the NSF. Interest in Dual IS-IS continued to grow, and most ISPs that sprung up in Europe chose to deploy ISO standards based on IS-IS instead of OSPF.

Unlike IS-IS which started as an ISO protocol, OSPF was inherently designed to support only IPv4 and was promoted by IETF as the referred IGP for IP networks. Additionally, because IS-IS support was not available on some major routers (noticeably Bay and 3com routers), OSPF automatically became the standard de-facto IGP for the reasonably large sized networks with multi-vendor platforms. An active IETF WG and evolving specifications also went a long way to help promote OSPF; and thus it started becoming more popular and more widely adopted compared to IS-IS.

There has been no major standardization effort in the ITU for a while, so ISO 10589 and RFC 1195 still remain the authoritative complete standards for IS-IS. The IETF IS-IS WG is now working on standardizing newer applications like MPLS, Traffic Engineering, IPv6, Multi-Topology Routing, HMAC-SHA authentication, etc for IS-IS.

To summarize, both the protocols have prevailed through the test of time and have established themselves as the IGPs of choice for ISPs. New extensions such as, MPLS TE, IPv6, have been deployed over the past 5+ years, and with active working groups for either protocol in IETF, they continue to evolve in lock-step fashion.

To cite a recent example, both the IS-IS and OSPF WGs are now working on defining HMAC-SHA authentication algorithms and procedures to increase the security associated with each one of these protocols. 


2 thoughts on “Genesis ..

  1. Excellent i can see you have a full and clear understanding of what is going on with ospf and isis esp on ispf and PRC. i combed th net for such clear and cute explanation and ended up in yours and that was best!


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