Cisco Certified Network Associate (CCNA) Practice Exam

Which port state in RSTP prevents the forwarding of frames?

1. Learning

2. Forwarding

3. Enabled

4. Discarding

The correct answer is: Discarding

In Rapid Spanning Tree Protocol (RSTP), the port state that prevents the forwarding of frames is the discarding state. This state is specifically designed to enhance the efficiency of the network by ensuring that no frames are sent or received on that port, allowing the switch to actively participate in the bridging process without creating loops. When a port is in the discarding state, it effectively ignores any incoming traffic, which is crucial for maintaining a loop-free topology in a switched network environment. This is particularly important in scenarios where there are redundant paths between switches, as it helps to eliminate potential broadcast storms or infinite loops. The discarding state is typically used during the initial stages of port activation (when a port transitions from listening to learning) or when a port is disabled due to a topology change or a higher priority configuration that dictates that it should not actively forward traffic. In contrast, other states, such as learning and forwarding, allow for frame processing, either to learn the addresses of devices or to send and receive frames, respectively. The enabled state is not a port state but rather indicates that a port is active and can transition between various states based on network conditions.