Santos:CCNA Sec 210-260 OCG

STP Is Wary of New Ports

When an interface is first brought up and receives a link signal from a connected device, such as a PC or router that is connected, STP is cautious before allowing frames in on the interface. If another switch is attached, there is a possible loop. STP cautiously waits for 30 seconds (by default) on a recently brought up port before letting frames go through that interface; 15 seconds of that is the listening state, where STP is seeing whether any BPDUs are coming in. During this time, it does not record source MAC addresses in its dynamic table. The second half of the 30 seconds (15 more) is then still looking for BPDUs, but STP also begins to populate the MAC address table with the source MAC addresses it sees in frames. ...