1.The routing table on the ISP network is very large. There are tens of thousands or even 100000 routes. Currently, there are more than 400000 routes on the global Internet. In the case of a large number of routes, IS-IS is more efficient than OSPF.
2.OSPF has more routing control functions than IS-IS. Standard IS-IS does not have the route filtering function. OSPF is more complex than IS-IS, for example, multiple areas, multiple protocol data packets, and multiple circuit types. In the case of the ISP, IS-IS is used only as an IGP that carries BGP. The IS-IS protocol is simple, stable, and scalable. IS-IS meets the preceding requirements. In addition, only IS-IS supports some advanced features on the MPLS network.
3.ISIS is based on TLV, and OSPF is not. One advantage of TLV is that it is easy to extend.