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.

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