The forwarding and learning port states exist in both traditional 802.1D Spanning Tree Protocol (STP) and 802.1w Rapid STP (RSTP). An STP switch port will pass through the following port states after a switch is turned on:
- Blocking
- Listening
- Learning
- Forwarding

When STP is enabled and a switch is turned on, each port first enters the blocking state. The switch port then transitions to the listening state, in which it begins processing bridge protocol data units (BPDUs) as it listens for information to determine whether it should transition to the learning state. After entering the learning state, a switch port begins to transmit BPDUs and learn addressing information with which to build the switching database. Finally, a switch port transitions to the forwarding state, in which the switch port forwards frames. If a switch port determines at any time during the STP state process that a switching loop would be caused by entering the forwarding state, the switch port enters the disabled state, in which the switch receives BPDUs but does not direct them to the system module.
RSTP combines the STP disabled, blocking, and listening states into a single port state called the discarding state. An RSTP switch port will pass through the following port states after a switch is turned on:
- Discarding
- Learning
- Forwarding