Interface authentication is configured with the isis password command. Intermediate System-to-Intermediate System (IS-IS) has three methods of authentication:
- Interface authentication
- Area authentication
- Domain authentication

Interface authentication configures IS-IS to add authentication for hello messages sentfrom that interface. Hello messages are used to establish and maintain adjacencies. To configure ISIS interface authentication, you should issue the isis password command from interface configuration mode. The syntax of the isis password command is isis password password [level1 | level2]. If the level is not specified, the password is used for both L1 and L2 routing. The isis password command is not used to configure router authentication.

Area authentication configures ISIS to add authentication for L1 Link State Packets (LSPs), Complete Sequence Number PDUs (CSNPs), and Partial Sequence Number PDUs (PSNPs). LSPs distribute routing information between nodes. The designated intermediate system (DIS), which is similar to the designated router (DS) in Open Shortest Path First (OSPF), sends CSNPs and PSNPs that describe the LSPs in the linkstate database. CSNPs, which describe all of the LSPs in the database, are multicast periodically by the DIS. PSNPs, which describe a subset of the LSPs, are used to acknowledge received LSPs and to request missing LSPs. To configure area authentication, you should issue the area password command from IS-IS router configuration mode. The syntax of the area password command is area password password.

Domain authentication configures IS-IS to add authentication for L2 LSPs, CSNPs, andPSNPs. To configure domain authentication, you should issue the domain password command from IS-IS router configuration mode. The syntax of the domain password command is domain password password.

It is possible to configure interface authentication, area authentication, and domain authentication on the same router.