This was supposed to be in the article but, I'll be honest, I wrote it over a vacation and stopped and started several times and some things got lost in the shu

This was supposed to be in the article but, I'll be honest, I wrote it over a vacation and stopped and started several times and some things got lost in the shuffle. I find IS-IS particularly interesting because, to summarize somewhat aggressively, TCP/IP first went into use basically without any working gateway protocols. So in early IP networks it was very common to use a full chunk of the OSI layer (IS-IS and CLNP) just to use IS-IS with IP prefixes shoehorned in, as the "IP" gateway protocol. OSPF is basically exactly this but formalized and running IS-IS over IP so you don't need the lower OSI layers. I'm sure there are still some IP networks out there today using the OSI stack for promulgating IP routes, it seems to still be well supported by all the vendors.