Two devices of ISIS are level-1, but the neighbor state is always down.
Log in the device and it is found that MTU configuration of both ends is the same, but MTU is small and it is smaller than default value of mtu-size set by global lsp-length originate. Change MTU as default one and the problem is solved.
1. The configuration is problematic.
2. The cooperation is problematic.
When enabling IS-IS under the interface, MTU of the interface must be larger than mtu-size of global lsp-length originate. Otherwise the interface regards MTU down. At the same time MTU of the interface must be larger than mtu-size of lsp-length receive, otherwise IS-IS packet cannot be correctly forwarded.