Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureHard
A network administrator is troubleshooting a Layer 2 loop in a complex enterprise network. The network uses Rapid PVST+ and has multiple redundant links. The administrator notices that some ports that should be blocking traffic are forwarding, leading to broadcast storms. Which STP enhancement is designed to prevent non-designated ports from transitioning to a forwarding state if they stop receiving BPDUs, thus protecting against loops?
- ARoot Guard
- BLoop Guard
- CBPDU Guard
- DPortFast
Show answer & explanationAnswer & explanation
Correct answer: B. Loop Guard
Loop Guard is an STP enhancement that prevents alternate or root ports from transitioning to a forwarding state if they stop receiving BPDUs. This protects against Layer 2 loops that can occur due to unidirectional links or software failures that cause a blocking port to incorrectly assume it should become designated and start forwarding.
Why the other options are wrong
- A. Root Guard prevents a port from becoming a root port, ensuring the desired root bridge election.
- C. BPDU Guard disables a port if it receives a BPDU, typically used on access ports to prevent unauthorized switches.
- D. PortFast immediately transitions an access port to the forwarding state, bypassing listening and learning, and is not for loop prevention in this context.
Loop Guard
An STP enhancement that prevents alternate or root ports from transitioning to a forwarding state if they stop receiving BPDUs, thereby protecting against Layer 2 loops.
- Prevents blocking ports from becoming forwarding.
- Monitors BPDU reception on non-designated ports.
- Protects against unidirectional link failures or software errors.
- Enhances STP stability and loop prevention.
Memory trick: Loop Guard: Guards against rogue forwarding where BPDUs disappear.