Tony Li wrote:

From a theoretical perspective, they both scale exactly the same. To wit: a two level hierarchy, with SPF within each level. Given ideal conditions, one could reasonably build a 160,000 router network with either protocol today without too much difficulty (400 nodes per area).

From a practical perspective, the implementation differences and topology issues dominate their practical applicability.

When one compares the complexity of the SPF calculation itself, both OSPF and ISIS scale in a similar way. The way routing information is packaged in LSAs and LSPs is different. They way those LSAs and LSPs are flooded is different too. I think these difference are more important when one compares scalability between ISIS and OSPF. Henk.