What happens to a port with RSTP PortFast enabled if it receives a BPDU (Bridge Protocol Data Unit)?
A) The port ignores the BPDU and remains in the forwarding state
B) The port is automatically disabled
C) The port transitions to the blocking state
D) The port is removed from the STP topology
Answer:
C) The port transitions to the blocking state
Option A: The port ignores the BPDU and remains in the forwarding state
Explanation:
This option is incorrect. When a port with RSTP (Rapid Spanning Tree Protocol) PortFast enabled receives a BPDU, it does not ignore the BPDU. Instead, the port reacts to the receipt of a BPDU as a safety measure to prevent potential network loops.
- Function of PortFast: RSTP PortFast is designed to be enabled on ports that are connected to end devices, such as workstations, printers, or servers. These devices do not participate in the Spanning Tree Protocol (STP) process and do not send or receive BPDUs. The primary purpose of PortFast is to allow these ports to immediately transition to the forwarding state without going through the usual STP states (blocking, listening, learning), which reduces the delay in connecting end devices to the network.
- Role of BPDUs: BPDUs are used by STP to exchange information between switches to prevent loops by determining which ports should be blocked and which should be placed in the forwarding state. If a PortFast-enabled port were to ignore BPDUs, it could inadvertently create a loop if the port was connected to another switch instead of an end device.
- Automatic Reaction: The receipt of a BPDU on a PortFast-enabled port indicates that the port might be connected to another switch or a network segment where loops are possible. To protect the network, the port must not ignore this BPDU but instead take action to prevent potential issues.
Thus, ignoring the BPDU and remaining in the forwarding state would be dangerous and is not the correct behavior for a PortFast-enabled port upon receiving a BPDU.
Option B: The port is automatically disabled
Explanation:
This option is incorrect. While it is true that receiving a BPDU on a PortFast-enabled port triggers a protective response, the port is not automatically disabled. Instead, the port undergoes a different process to ensure the network’s stability.
- Port Disabling vs. Blocking: When a port is disabled, it is completely shut down and stops all traffic, both inbound and outbound. This is a more extreme action than what typically happens when a PortFast-enabled port receives a BPDU.
- Blocking State: The correct reaction is for the port to transition to the blocking state, not to disable itself entirely. In the blocking state, the port stops forwarding traffic but remains active and can be brought back to the forwarding state if network conditions change.
- RSTP and PortFast Behavior: RSTP is designed to maintain network stability while allowing for rapid reconfiguration if a topology change is detected. Disabling the port would be a drastic measure and is not the intended response for a PortFast-enabled port receiving a BPDU.
Thus, while disabling the port would certainly prevent loops, it is not the standard or correct behavior in this scenario. The port should transition to a blocking state instead, which still allows it to participate in the STP process and potentially re-enable forwarding if it is safe to do so.
Option C: The port transitions to the blocking state
Explanation:
This option is correct. The appropriate action for a PortFast-enabled port when it receives a BPDU is to transition to the blocking state. This mechanism is in place to protect the network from loops.
- Transition to Blocking State: The blocking state is one of the key states in STP. When a port is in the blocking state, it does not forward any data frames, effectively preventing any possible loops from occurring. By transitioning to this state upon receiving a BPDU, the port ensures that it will not contribute to a loop.
- Automatic Deactivation of PortFast: When a BPDU is received, PortFast is automatically disabled, and the port begins to operate as a regular STP port. This means it will participate in the STP process, potentially moving through the listening and learning states before returning to forwarding, if no loop risk is detected.
- Importance of This Mechanism: This safety mechanism ensures that even if a network administrator mistakenly enables PortFast on a port connected to another switch, the network can still protect itself. The receipt of a BPDU triggers the transition to the blocking state, preventing the port from creating a loop.
- Network Stability: This behavior is crucial for maintaining network stability. Loops can cause severe issues, such as broadcast storms, where frames circulate endlessly in the network, consuming bandwidth and processing power. By transitioning to the blocking state, the port helps to prevent these issues.
Therefore, the correct response of a PortFast-enabled port receiving a BPDU is to transition to the blocking state, making this the correct option.
Option D: The port is removed from the STP topology
Explanation:
This option is incorrect. When a port with RSTP PortFast enabled receives a BPDU, it is not removed from the STP topology. Instead, it becomes an active participant in the STP process to prevent loops.
- STP Topology: The STP topology includes all the ports that are part of the spanning tree, whether they are in the blocking, listening, learning, or forwarding state. Removing a port from the topology would mean that the port is no longer considered part of the network’s loop prevention strategy, which is not the intended action when a BPDU is received.
- Participation in STP: Upon receiving a BPDU, the port that had PortFast enabled transitions to the blocking state and begins to function as a normal STP port. This means it is still part of the STP topology and will follow the standard STP rules, potentially moving to the forwarding state if it is safe to do so.
- No Removal Process: There is no process within RSTP or STP that would remove a port from the topology as a response to receiving a BPDU. The correct behavior is for the port to participate in the STP process to help maintain network stability.
Thus, removing the port from the STP topology is not the correct action. Instead, the port should remain part of the STP process and transition to the blocking state if necessary.
Conclusion
The correct action for a port with RSTP PortFast enabled when it receives a BPDU is to transition to the blocking state (Option C). This behavior ensures that the port does not inadvertently create a network loop, which could cause significant network disruptions. Other options, such as ignoring the BPDU, disabling the port, or removing it from the STP topology, are incorrect and do not align with the intended protective mechanisms of RSTP PortFast. By transitioning to the blocking state, the port helps maintain the stability and integrity of the network, allowing STP to perform its essential role in loop prevention.