Which two port states are considered 'stable' in STP?

The two port states considered 'stable' in Spanning Tree Protocol (STP) are blocking and forwarding.

In STP, a port in the forwarding state can send and receive data frames, which allows normal network traffic flow. This is a stable state as it indicates that the port is actively participating in the network.

The blocking state, on the other hand, is also a stable state because the port will not send or receive frames but is still aware of the network topology. A port in this state can transition to the forwarding state when necessary to prevent network loops, indicating that it is functioning and maintaining its role when called upon.

In contrast, the listening and learning states are transitional states. During the listening state, the port prepares to forward data by learning MAC addresses but does not yet forward frames, while in the learning state, the port continues to learn MAC addresses but does not forward frames until it transitions to the forwarding state. Both these states are essential for the operation of STP but are not considered stable since they do not allow traffic flow.