Why does an IS-IS level-1 neighbor relationship fail to be set up

Q: Why does an IS-IS level-1 neighbor relationship fail to be set up?

A: An IS-IS Network Entity Title (NET) of a switch is in the X…X.XXXX. XXXX.XXXX.00 format. X…X indicates the area address, the 12 Xs in the middle indicate the system ID of the switch, and 00 indicates the selector (SEL).
The area address length ranges from 1 bytes to 13 bytes. Switches in the same area must be configured with the same area address.
The 6-byte system ID of a switch is unique in the entire area and backbone area.
The SEL is the last byte and its value must be 00.
IS-IS devices can set up a level-1 neighbor relationship only when the following conditions are met:
1. The devices have the same area address.
2. The devices have different system IDs.
3. The SEL of the devices is 00.
If any of the preceding conditions is not met, the level-1 neighbor relationship cannot be set up.

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