In the process of selecting an IGP to run on a large scale nationwide IP Network, I received many responses to my NANOG request for information on IS-IS versus OSPF protocols. Most of the resources mentioned in the responses, however, were bias towards OSPF. There was not much information on IS-IS to be found, although obviously some individuals are satisfied enough with this protocol that is it still in place in large networks. Thank you to everyone who responded to my request. Rather than responding to each of you individually, I have summarized the information I received below (although admittedly one-sided). General: "The two are really pretty much equivalent, other than that IS-IS is intended for OSI networks while OSPF is for IP networks. Then there's "Integrated IS-IS" that handles both." Politics: "In fact, a number of members in the OSPF working group feared that IS-IS would be preferred for "political" reasons, as part of a grand plan to convert the Internet to OSI correctness, even if that meant a complete disruption of the service."(1) "There were also non-technical considerations. Many people felt that it was better that the IETF have complete control over the OSPF protocol design rather than depend on an ISO committee whose goals, namely to produce a routing protocol for the OSI protocol stack, were somewhat different."(2) Technical Issues: The following are excerpt from John Moy's book discusses the reasons that the IETF went ahead with the development of OSPF even though IS-IS was already being developed: "First, IS-IS ran directly over the link layer, which we thought was the wrong choice for a TCP/IP routing protocol..." "IS-IS had an area routing scheme, but one that did not allow any shortcuts between areas that we thought were necessary for an Internet routing protocol." "Also, IS-IS made no attempt to align fields in their packet formats, making life more difficult for protocol implementers." "It is possible to run IS-IS on an IP-only network, but, even if one does not want to forward CLNP datagarms, one will have to install in the routers the low-level code that demultiplexes CLNP and IS-IS packets. The "area" model of IS-IS is designed for CLNP where the rigidity and connectivity requirements are somewhat compensated by the possibility to perform automatic address assignment; when one uses IS-IS for IP, the rigidity remains, but the advantages disappear. IS-IS suffers from at least two other problems; a tiny metric and a limit to what a router can advertise. Having only 6 bits to express a link's metric is not really a good idea. It may make some messages shorter, but it certainly diminishes the routing's precision. Then, using the 8-bit link state record number to identify the link state record's will limit to 256 the number of records that a given router can advertise. As the size of each record is limited to the maximum packet size supported by the network, this can prove to be a severe constraint."(1) Dr. Huitema goes on to state some advantages of OSPF over IS-IS: * Support of a backup designated router * Coexistence with RIP through "not-so-stubby-areas" [Okay, this ones a little dated] * OSPF is "change controlled" by the IETF, evolution of OSI Protocols was much slower and that it responded to many other forces besides user needs. Ron Johnson wrote: OSPF: Uses a multicast address and standard IP transport to carry it's packets. IS-IS: Uses it own transport method to carry it's packets. OSPF: Scales well when the Backbone/area/stub model is correctly implemented, otherwise with many routers in a single area, it can have a definite performance impact on routing convergence times. IS-IS: Is less sensitive to growth problems then OSPF. OSPF: Is easier to implement and understand. It is more widely used, and supported amongst many vendors. IS-IS: Is more arcane. Harder to implement and understand it's operation and behavior, and may not be supported on every router. So, the bottom line, for me is; Use OSPF V2, if you are building a network where you can create a "backbone" area and then other logical areas attached to the backbone. Then aggregate and summarize your route announcements in your areas and announce the summarized routes into the backbone area. You will end up with less routes in your IGP. Which in turn means lower OSPF recalculation times, better performance and a more stable IGP. References: (1)"Routing and the Internet" By: Christian Huitema (ISBN: 0131321927) (2)"OSPF Anatomy of an Internet Routing Protocol" By: John Moy (ISBN: 0201634724) "Routing TCP/IP Volume I" By: Jeff Doyle, CCIE (ISBN:1578700418) RFC2178 "OSPF Version 2" (http://www.faqs.org/rfcs/rfc2178.html) RFC1195 "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments" (http://www.faqs.org/rfcs/rfc1195.html) RFC1371 "Choosing a "Common IGP" for the IP Internet (The IESG's Recommendation to the IAB)" (http://www.faqs.org/rfcs/rfc1370.html) Previous NANOG Discussions on this topic: http://www.cctec.com/maillists/nanog/historical/9810/threads.html#00484 Other References I have yet to review: "Interconnections: Bridges and Routers" By: Radia Perlman (ISBN: 0201563320) "The Great IGP Debate--Part One:IS-IS and Integrated Routing." By: R. Perlman and R. Callon (ConneXions 5, no. 10, October 1991) "The Great IGP Debate--Part Two:The Open Shortest Path First (OSPF) Routing Protocol." (ConneXions 5, no. 10, October 1991) Dan Rabb Network digital broadcast network dan@dbn.net