why we use IS-IS?
IS-IS is suppose to be perfect protocol for large environment, LAN and Point to point environment. Like the OSPF protocol intermediate system to intermediate system (IS-IS) is also a link state routing protocol with fast convergence. ISIS uses the OSI protocols and CLNP address for communication among each other.
- IS-IS also has the area concept but its areas are different from OSPF, in ISIS world IS (intermediate system) is a router and there are L1, L2, L1/L2 router in IS-IS . L1 router only talks to L1 router and L2 to L2, L1 is a non backbone router like in OSPF.
- IS-IS is a not a good choice for NBMA (non broadcast multi access) or point to multi-access like frame relay, in such type of environment EIGRP is good choice. Also for dialup connections OSPF is a best choice instead of IS-IS.
You can learn more about IS-IS