Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureHard
A network engineer is troubleshooting a Layer 2 loop in a complex enterprise network. The loop is causing broadcast storms and MAC address table instability. The network uses Rapid PVST+. Which STP feature is designed to prevent a port from becoming a root port or designated port if it receives its own BPDU or a superior BPDU, effectively protecting against accidental loops caused by misconfigurations?
- ABPDU Guard
- BLoop Guard
- CRoot Guard
- DPortFast
Show answer & explanationAnswer & explanation
Correct answer: B. Loop Guard
Loop Guard is an STP enhancement that prevents alternate or root ports from becoming designated ports due to a unidirectional link failure or software error that causes BPDUs to stop being received on a segment. If BPDUs are no longer received on an STP-enabled port that was previously forwarding, Loop Guard transitions the port into a loop-inconsistent state, blocking traffic and preventing a loop.
Why the other options are wrong
- A. BPDU Guard shuts down a PortFast-enabled port if it receives any BPDU, preventing unauthorized switches from joining the STP topology.
- C. Root Guard prevents a port from becoming a root port, ensuring the desired root bridge remains in control of the STP topology.
- D. PortFast immediately brings a port to the forwarding state, used for end devices, and can create loops if connected to other switches without BPDU Guard.
Loop Guard
An STP feature that prevents alternate or root ports from transitioning into a forwarding state if they stop receiving BPDUs, thereby protecting against Layer 2 loops caused by unidirectional link failures or software errors.
- Prevents ports from becoming designated and causing loops.
- Monitors BPDU reception on non-designated ports.
- Transitions port to loop-inconsistent (blocking) state if BPDUs cease.
Memory trick: Loop Guard: Lost BPDUs, Locked Down