IS-IS adds an additional TLV record to LSPs to facilitate the authentication password that’s used. Since LSPs cannot be modified by any router other than the originating router, all routers within an area must have the same name configured.
LSP Authentication passwords are separated by LSP type. L1 LSPs can have one password while using a different password for L2 LSPs. You can manually define these passwords using the respective IOS commands for each LSP type.
You can configure authentication to be in IIH LSPs, but you can also enable authentication on CSNPs, and PSNPs but configuring the password under the IS-IS process itself.
If passwords do not match this will force adjacencies to fail. Authentication passwords can be set to clear text or MD5.