•OSPF supports NBMA and point-to-multipoint links, IS-IS does not.
•IS-IS rides directly above layer two, versus on IP like OSPF, which may offer a security advantage (IS-IS attacks cannot be routed).
•OSPF can support virtual links, IS-IS cannot (because it rides L2 directly).
•On broadcast networks, OSPF elects a DR and BDR which cannot be preempted, whereas IS-IS elects only a single DIS which may be preempted.
•OSPF designates a backbone area (area 0) for inter-area advertisements; IS-IS organizes the domain into two layers.
•OSPF routers can belong to multiple areas, IS-IS routers belong to exactly one area.
•OSPF has more strict requirements for forming neighbor adjacencies. The hello and dead intervals must match, and the subnet mask must match (except on point-to-point links).
Of course there any many more differences discussed in the paper, some of which should be weighed carefully when deciding between an OSPF and an IS-IS deployment.
