Three-way handshake is implemented on P2P interfaces of IS-IS devices. Why is the local switch Up and the peer switch down

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Q: Three-way handshake is implemented on P2P interfaces of IS-IS switches. Why is the local switch Up and the peer switch down?
A: To be compatible with the peer switch supporting only two-way handshake rather than three-way handshake, the local switch also works in two-way handshake mode. As a result, there is a possibility that the local switch is Up and the remote switch is Down.

Other related questions:
An interface on a WLAN device goes Up or Down frequently
The possible causes are as follows: - The power cable is faulty or the optical fiber does not meet requirements. - The duplex modes, rates, and negotiation modes are different on the connected interfaces. - The device hardware is faulty. Perform the following operations to rectify the fault: 1. If the interface is an electrical interface or a combo interface working in electrical mode, check whether the power cable is faulty and securely connected on the local and remote devices. If the interface is an optical interface or a combo interface working in optical mode, check whether the type and length of the optical fiber, and the transmission distance supported by the optical module meet requirements. 2. Run the display interface [ interface-type [ interface-number ] ] command to check the rate, duplex mode, and negotiation mode of the interface. In the command, interface-type and interface-number specify the interface type and number, respectively. - Check the Negotiation field. If ENABLE is displayed, the interface works in auto-negotiation mode. If DISABLE is displayed, the interface works in non-auto-negotiation mode. The two interfaces on the local and remote devices must work in the same negotiation mode (auto-negotiation or non-auto-negotiation mode). You can run the negotiation auto command in the interface view to configure an interface to work in auto-negotiation mode. If the fault persists, configure the interface to work in non-auto-negotiation mode, and forcibly set the same rate and duplex mode for the interconnected interfaces. - Check the Speed field. If the rates of the interfaces on the local and remote devices are different, run the speed command in the interface view to set the same rate for the two interfaces. - Check the Duplex field. If the duplex modes of the interfaces on the local and remote devices are different, run the duplex command in the interface view to set the same duplex mode for the two interfaces. 3. Perform a loopback test on the AC using an Ethernet cable to connect the interface to another interface. If the interface can go Up, the WLAN is working properly. The remote device may be faulty. Contact the vendor of the remote device to rectify the fault. - If an electrical interface or a combo interface working in electrical mode cannot go Up, change it to another interface. If an optical interface or a combo interface working in optical mode cannot go Up, replace the optical module.

There is a delay when a fixed switch interface alternates between Up and Down consecutively. Why
To prevent the interface from frequently reporting the status change event, run the carrier { up-hold-time | down-hold-time } interval command to set the delay in reporting the status change event. By default, the delay in reporting a port Up event is 2000 milliseconds, and the delay in reporting a port Down event is 0 milliseconds.

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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