The Shortest Path First (SPF) algorithm used by all (read: both) popular link-state routing protocols should be well-known to all network engineers, but the fact that the IP routing table gets populated by a distance-vector-like second phase of the algorithm is oft forgotten.
Check this short presentation in case you’d like to get a bit more in-depth explanation of the SPF algorithm.
The second phase of the link state route selection algorithm is called Partial SPF in OSPF and Partial Route Calculation (PRC) in IS-IS. Obviously it’s beneficial if the router can react to a change in the network topology by running PRC, not the full SPF, and there are significant differences in the way OSPF and IS-IS respond to various topology change events. The differences are not architectural; you could make OSPF behave as well as IS-IS. They just prove that the old wisdom is still true: very large IOS-based networks use IS-IS because it’s better implemented than OSPF.
I’ve already described the problems you’re facing when running IS-IS over partially meshed Frame Relay and provided sample working configurations. To conclude this topic, I’ve also written a “technical solution” document that describes the symptoms, verification steps and the necessary configuration changes.
In the IS-IS on multi-access partially-meshed Frame Relay interface article I’ve described the design rules you have to follow when implementing IS-IS over partially-meshed multi-access network. The IS-IS on partially-meshed Frame Relay subnet: sample configuration article contains step-by-step example, including initial router configuration, IS-IS configuration, verification steps and complete final configurations.
If someone insists on running IS-IS on a partially-meshed Frame Relay network without using subinterfaces (which is the proper way of doing it), it can still be done … assuming you understand what’s going on and properly configure the IS-IS routers.
A member of NIL’s forums wanted to run IS-IS over a hub-and-spoke Frame Relay network without using subinterfaces. I hope the question is not related to a production network; running IS-IS over a generic partially-meshed multi-access WAN network is not a good idea.