Which state is used by a port to transition from discarding to learning in RSTP?

In the context of Rapid Spanning Tree Protocol (RSTP), the state used by a port to transition from discarding to learning is indeed the learning state. RSTP optimizes the traditional Spanning Tree Protocol (STP) by enabling faster convergence and defining port states to facilitate this process.

When a port is in the discarding state, it does not forward frames and does not learn MAC addresses. The transition to the learning state indicates that the port is now beginning to learn MAC addresses from the traffic it sees, though it still does not forward frames at this point. This is a crucial step in allowing the network to populate the MAC address table, which prepares the port for the next state—forwarding—where it can actively participate in traffic forwarding.

The learning state ensures that the switch is aware of the devices attached to the network before it starts forwarding traffic, thus preventing potential loops and ensuring efficient network operation. This careful progression through these states is part of RSTP's design to enhance overall network performance and stability.