Why is an unconnected interface Up

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The loopback internal command may be configured on the interface to enable internal loopback detection. Therefore, the interface is Up.

You can run the display interface gigabitethernet 0/0/1 command in any view to check whether internal loopback detection is enabled on GE0/0/1.
display interface gigabitethernet 0/0/1
...
Port Mode: FORCE COPPER
Speed : 1000, Loopback: INTERNAL
Duplex: FULL, Negotiation: ENABLE
Mdi : AUTO, Flow-control: DISABLE
...The command output shows that internal loopback detection is enabled on the interface.

Other related questions:
Why Is a Non-Connected Interface Up?
The loopback internal command may be configured on the interface to enable internal loopback detection. Therefore, the interface is Up. You can run the display interface gigabitethernet 1/0/1 command in any view to check whether internal loopback detection is enabled on GE1/0/1. display interface gigabitethernet 1/0/1 ... Port Mode: FORCE COPPER Speed : 1000, Loopback: INTERNAL Duplex: FULL, Negotiation: ENABLE Mdi : AUTO, Flow-control: DISABLE ... The command output shows that internal loopback detection is enabled on the interface.

Why cannot an Up interface be selected as the working interface
The reason is that this interface works in half duplex mode. On a switch, if the rate of an interface is the same as that of member interfaces in an Eth-Trunk, this interface can be added to the Eth-Trunk. However, if this interface works in half duplex mode, the Eth-Trunk will not select it as the working interface.

Why is the dialer interface still Up when the physical status of the dialer interface is Down
When the physical interface that is not connected to a cable is Down, the dialer interface displays Snoofing Up. This situation is normal. You can run the dialer number 1 autodial command so that the dialer interface changes as the physical interface.

Why dial-ups through a 3G Cellular interface always fail
The problem is caused by the following reasons in general: - The 3G data card is not recognized by the network, or the 3G data card is not included in the supported data card list of the device. - Some authentications or negotiations fail during the dial-up process because the 3G data card does not match local networks, resulting in dial-up failures. (For example, some special authentications may need to be performed in local networks, if the authentications are not enabled on a 3G data card or a USIM/SIM card, the card does not match the local networks.) - The 3G data card is not registered to an available network, manifesting as poor signal strength. You can run the display cellular command to check the status of 3G signal. In the displayed output information, the value of the Current Service Status field must be Service available, and the signal strength must be normal. - The mode of the 3G data card is not properly configured. For example, The network connection method of a 3G data card in which a SIM card is inserted is set to wcdma-only. - The USIM/SIM card is not available. - The status of the USIM/SIM card is abnormal. You can run the display cellular command to check whether the status of the USIM/SIM card is normal. The status of the USIM/SIM card must be OK but not Not insert. - The USIM/SIM card does not subscribe to 2G/3G services (services are not subscribed). - The fund of the USIM/SIM card is insufficient. - Check whether the 3G configurations are correct, including the dial-up character string. If you want to dial up to the CDMA2000 network, confirm whether a PPP authentication needs to be performed to access the network. - The profile is not configured or incorrectly configured. - Check whether you disable the 3G Cellular interface by running the shutdown command.

Why does the AR router interface change between Up and Down
When auto-negotiation is enabled on one end but disabled on the other end, set the same auto-negotiation mode on both ends. If auto-negotiation is enabled on one end of an electrical interface but disabled on the other end, packets may be discarded even when the interface is Up.

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