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

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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.

Other related questions:
On a square-shaped network, OSPF neighbor relationships cannot be set up on P2P interfaces of S series switches. Why
Question: Four devices in the same VLAN form a square-shaped network and the type of their interfaces is P2P. Why cannot the devices establish OSPF neighbor relationships? Answer: A Hello packet, as a multicast packet, sent by a device is transmitted within the entire VLAN since interfaces of the devices are in the same VLAN. P2P interfaces do not check masks, so when another device receives the Hello packet, it immediately establishes a neighbor relationship. However, the device neglects Hello packets sent by the other two devices since a P2P interface can only establish one neighbor relationship at a time. This causes the neighbor status of each device remaining Init. A sends a Hello packet, and B receives the packet �?A considers B as its neighbor. B sends a Hello packet, and C receives the packet �?B considers C as its neighbor. C sends a Hello packet, and D receives the packet �?C considers D as its neighbor. D sends a Hello packet, and A receives the packet �?D considers A as its neighbor. Therefore, the devices cannot establish Full OSPF neighbor relationships.

Why can a user use commands higher than level 1 after the user level configured on the RADIUS server for the user is set to level 1
Run the display current-configuration command to check the configuration on the router and check whether the command line level has been changed by running the command-privilege level command.

Why does the state of the MSDP peer keep down though the MSDP peer relationship is set up
You can run the peer peer-address connect-interface interface-type interface-number command on two ends to set up an MSDP peer relationship. The address of the interface specified by interface-type interface-number in the locally configured peer connect-interface command must be consistent with peer-address specified in the peer connect-interface command run on the remote end.

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