Why cannot an Up interface be selected as the working interface

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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.

Other related questions:
Why cannot STP work in the 30 seconds after an interface is Up
STP slow convergence is performed at an interval of 30 seconds. Check whether this problem is caused by STP slow convergence.

Why is an unconnected 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 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.

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 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.

The state of an interconnected SA interface cannot be Up
Problem description: The state of the physical interface and protocol of the SA card cannot be Up. Serial1/0/0 current state: DOWN Line protocol current state: DOWN Description: HUAWEI, AR Series, Serial1/0/0 Interface Route Port,The Maximum Transmit Unit is 1500, Hold timer is 10(sec) Internet Address is 172.20.18.22/30 Link layer protocol is nonstandard HDLC Last physical up time : 2012-08-09 16:04:29 Last physical down time : 2012-08-09 16:04:29 Current system time: 2012-08-09 16:32:47 Physical layer is synchronous, Virtualbaudrate is 64000 bps Interface is DTE, Cable type is V24, Clock mode is TC Last 300 seconds input rate 0 bytes/sec 0 bits/sec 0 packets/sec Last 300 seconds output rate 0 bytes/sec 0 bits/sec 0 packets/sec Handling process: 1. If the peer end does not receive the signal, run the following command to set the transmit clock signal of the serial interface in inversion synchronization mode: [Huawei-Serial1/0/0]invert transmit-clock 2. If the local end does not receive the signal, run the following command to set the receive clock signal of the serial interface in inversion synchronization mode. [Huawei-Serial1/0/0]invert receive-clock Note: Clock signals of the serial interface in inversion synchronization mode can be configured only when AR3200 is used as a DTE device. After the preceding steps, most of the problems that the state of the SA interface is not Up can be resolved or filtered out. Root cause: The clock phases of the peer devices connected through the SA interface are not consistent.

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