IS-IS is one of the interior gateway protocol, widely used by telecom operators. There are many differences between Link State Database synchronization and routing computation and OSPF. This paper studies on the IS-IS protocol from the perspective of neighbor relationship establishment and synchronization, and compares it with OSPF protocol, trying to clarify the principle and pro- cess of Link State Database synchronization and routing calculation of IS-IS protocol, which has certain reference for the study of IS- IS routing protocol.