When does FR enter the Up state

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FR enters the Up state when Local Management Interface (LMI) negotiation succeeds. You can run the display fr interface command to view FR status on an interface.

Other related questions:
What it means when the state of the FR link protocol is Up
It means that the negotiation of the Local Management Interface (LMI) protocol is successful. You can run the display fr interface command to check the FR protocol information about the interface.

Why an interface does not enter the linkdown state when its receiving power reaches the lower threshold
An enhanced optical module has two thresholds for optical power: a warning threshold and an alarm threshold. When the receiving power of an interface falls below the lower warning threshold, packets may be lost on the interface, but the interface does not enter the linkdown state. When the receiving power of an interface reaches the lower alarm threshold, severe packet loss may occur, or even the interface enters the linkdown state. A high receiving power may corrupt an optical module.

The state of an optical interface is not Up when it interconnects with the peer device
For the problem that the state of an optical interface is not Up when it interconnects with the peer device, see the troubleshooting guide. For details, access the URL in the right column.

On S series switches, when BFD sessions in Down state receive packets in Up state sent from the peer end, how does the state change
S series switches with their BFD sessions in Down state do not change the status of BFD sessions to Up after receiving BFD Up packets from the remote end. If the link between two communication ends encounters an instant disconnection after the two ends become Up, one end that first detects the link Down event sends a BFD Down packet to the remote end. The remote end sends a BFD Up packet to the local end before receiving the BFD Down packet. The local end does not change its state after receiving the Up packet. This prevents network flapping.

The state of an E1 interface is not Up when the interface interconnects with the peer device
For the problem that the state of an E1 interface of an AR router is not Up when the interface interconnects with other devices, see the troubleshooting guide. For details, access the URL in the right column.

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