Which option is the protocol that will prevent a bridging loop if BPDUs are received on an interface that has the STP calculation disabled?
- RSTP
- BPDU guard
- PortFast
- MST
Explanation: BPDU guard is a Cisco enhancement used to prevent bridging loops on ports where traditional Spanning Tree Protocol (STP) calculations have been effectively bypassed using PortFast.
When PortFast is enabled on an access port, the port transitions immediately from blocking to forwarding, skipping the standard listening and learning states to allow end devices to connect quickly. Because these ports are intended only for end-user devices (like PCs or printers) that do not send BPDUs, receiving a BPDU on such an interface would indicate that another switch or bridge has been connected. If left unchecked, this could create a catastrophic bridging loop.
BPDU guard protects the network by monitoring these interfaces; if it detects an incoming BPDU, it immediately shuts down the port and places it into an errdisable state. This ensures that the STP domain borders are enforced and the network topology remains predictable and loop-free.
Related exam: Modules 1 – 7: Checkpoint Exam: Network Switching Answers (CCNP ENCOR v9)
