Cisco – Enable PortFast on the port.

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PortFast should only be enabled on ports connected to a single hosts connecting hubs, concentrators, switches,  bridges, etc.to this interface when PortFast is enabled, can cause temporary spanning tree loops. Use with CAUTION

SwitchB#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SwitchB(config)#interface fastethernet 5/4
SwitchB(config-if)#spanning-tree portfast
%PortFast has been configured on FastEthernet5/4 but will only have effect when the interface is in a non-trunking mode.