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. This is called the learning state. After listening and learning have completed (full 30 seconds), the switch can begin forwarding frames. If a port is in a blocking state at first, an additional 20-second delay might occur as the port determines that the parallel path is gone, before moving to listening and learning.

For most administrators and users, this delay is too long. When configured, enhancements to STP, including the PortFast feature, can tell the switch to bypass the listening and learning stage and go right to forwarding. This leaves a small window for a loop if a parallel path is injected in the network.