CCNA Switching and Network Access Practice Question
Network Topology
You are connected to R1, a multilayer switch acting as the STP root for VLAN 10. Configure Root Guard on port GigabitEthernet0/1 (designated port) to protect against superior BPDUs from an unauthorized switch, Loop Guard on uplink GigabitEthernet0/2 to prevent forwarding loops on unidirectional links, and BPDU Guard on PortFast-enabled GigabitEthernet0/3 to shut down the port if a BPDU is received. After configuration, troubleshoot the scenario: a superior BPDU is received on G0/1, causing it to be blocked by Root Guard, and an unauthorized switch sends a BPDU to G0/3, placing it in err-disable state. Verify the final configuration and state.
⚠ Common exam trap
Watch out for confusion between Root Guard and BPDU Guard states: Root Guard causes root-inconsistent (BKN*), while BPDU Guard causes err-disable. Also, remember Root Guard is for designated ports, not root ports. Loop Guard can be applied to any port expecting BPDUs, not just root ports.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
[CORRECT] Root Guard on G0/1 is correctly configured; when a superior BPDU is received, the port is placed into a root-inconsistent state (BKN* in show spanning-tree) to prevent the switch from becoming root. Loop Guard on G0/2 prevents loops if BPDUs stop arriving due to a unidirectional link. BPDU Guard on G0/3, combined with PortFast, err-disables the port upon receiving any BPDU, as shown by the err-disabled status. To recover, the administrator must manually re-enable the interface after removing the offending device. No additional configuration is required; the existing commands are correct and produce the expected behavior.
The scenario demonstrates three STP protection mechanisms. Root Guard on G0/1 is correctly configured; when a superior BPDU is received, the port is placed into a root-inconsistent state (BKN* in show spanning-tree) to prevent the switch from becoming root. Loop Guard on G0/2 prevents loops if BPDUs stop arriving due to a unidirectional link. BPDU Guard on G0/3, combined with PortFast, err-disables the port upon receiving any BPDU, as shown by the err-disabled status. To recover, the administrator must manually re-enable the interface after removing the offending device. No additional configuration is required; the existing commands are correct and produce the expected behavior.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
[CORRECT] Root Guard on G0/1 is correctly configured; when a superior BPDU is received, the port is placed into a root-inconsistent state (BKN* in show spanning-tree) to prevent the switch from becoming root. Loop Guard on G0/2 prevents loops if BPDUs stop arriving due to a unidirectional link. BPDU Guard on G0/3, combined with PortFast, err-disables the port upon receiving any BPDU, as shown by the err-disabled status. To recover, the administrator must manually re-enable the interface after removing the offending device. No additional configuration is required; the existing commands are correct and produce the expected behavior.
Why this is correct
Ly describes the behavior of Root Guard, Loop Guard, and BPDU Guard as per Cisco IOS. Root Guard forces a port to become a designated port and blocks it if a superior BPDU is received, placing it in a root-inconsistent state (BLK*). Loop Guard prevents loops by putting the port into loop-inconsistent state if BPDUs stop. BPDU Guard err-disables a PortFast-enabled port upon BPDU reception. Manual intervention is required to recover from err-disable.
- ✗
Root Guard on G0/1 is incorrectly configured; it should be configured on the root port, not the designated port. Loop Guard on G0/2 is correctly configured. BPDU Guard on G0/3 is correctly configured, but the port should automatically recover from err-disable state after a timeout.
Why it's wrong here
This is incorrect because Root Guard is specifically designed to be configured on designated ports to prevent them from becoming root ports. Configuring it on a root port would be ineffective. Also, BPDU Guard does not automatically recover; the port remains err-disabled until manually re-enabled or configured with errdisable recovery.
- ✗
Root Guard on G0/1 is correctly configured. Loop Guard on G0/2 is incorrectly configured because Loop Guard should be applied to root ports, not uplink ports. BPDU Guard on G0/3 is correctly configured, but the port should be in a blocking state, not err-disabled.
Why it's wrong here
This is incorrect because Loop Guard can be applied to any port that is a potential root port or alternate port, but it is commonly used on blocked ports or uplinks. The key error is that BPDU Guard causes err-disable, not a blocking state. Also, the statement about Loop Guard being only for root ports is inaccurate.
- ✗
Root Guard on G0/1 is correctly configured. Loop Guard on G0/2 is correctly configured. BPDU Guard on G0/3 is incorrectly configured because BPDU Guard should be applied to trunk ports, not access ports, and the port should be placed in a root-inconsistent state.
Why it's wrong here
This is incorrect because BPDU Guard can be applied to any PortFast-enabled port, regardless of whether it is an access or trunk port. The port is err-disabled, not placed in a root-inconsistent state (which is a Root Guard behavior).
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓[CORRECT] Root Guard on G0/1 is correctly configured; when a superior BPDU is received, the port is placed into a root-inconsistent state (BKN* in show spanning-tree) to prevent the switch from becoming root. Loop Guard on G0/2 prevents loops if BPDUs stop arriving due to a unidirectional link. BPDU Guard on G0/3, combined with PortFast, err-disables the port upon receiving any BPDU, as shown by the err-disabled status. To recover, the administrator must manually re-enable the interface after removing the offending device. No additional configuration is required; the existing commands are correct and produce the expected behavior.Correct answer▾
Why this is correct
Ly describes the behavior of Root Guard, Loop Guard, and BPDU Guard as per Cisco IOS. Root Guard forces a port to become a designated port and blocks it if a superior BPDU is received, placing it in a root-inconsistent state (BLK*). Loop Guard prevents loops by putting the port into loop-inconsistent state if BPDUs stop. BPDU Guard err-disables a PortFast-enabled port upon BPDU reception. Manual intervention is required to recover from err-disable.
✗Root Guard on G0/1 is incorrectly configured; it should be configured on the root port, not the designated port. Loop Guard on G0/2 is correctly configured. BPDU Guard on G0/3 is correctly configured, but the port should automatically recover from err-disable state after a timeout.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: Root Guard is applied to designated ports, not root ports. BPDU Guard does not auto-recover by default.
Why candidates choose this
Candidates may confuse Root Guard with Root Guard placement or assume BPDU Guard has automatic recovery similar to UDLD.
✗Root Guard on G0/1 is correctly configured. Loop Guard on G0/2 is incorrectly configured because Loop Guard should be applied to root ports, not uplink ports. BPDU Guard on G0/3 is correctly configured, but the port should be in a blocking state, not err-disabled.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: Loop Guard is not restricted to root ports; it can be used on any port where BPDUs are expected. BPDU Guard results in err-disable, not blocking.
Why candidates choose this
Candidates may think Loop Guard is only for root ports because it is often used there, or confuse BPDU Guard's action with normal STP blocking.
✗Root Guard on G0/1 is correctly configured. Loop Guard on G0/2 is correctly configured. BPDU Guard on G0/3 is incorrectly configured because BPDU Guard should be applied to trunk ports, not access ports, and the port should be placed in a root-inconsistent state.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: BPDU Guard is not limited to access ports; it works on any PortFast-enabled port. The state is err-disable, not root-inconsistent.
Why candidates choose this
Candidates may associate BPDU Guard with access ports only, or confuse the err-disable state with root-inconsistent state from Root Guard.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
PortFast
PortFast is a Cisco switch feature that immediately brings a port into the forwarding state, bypassing the normal Spanning Tree Protocol (STP) listening and learning phases, so that devices connected to that port can start communicating right away.
Key term
BPDU
A Bridge Protocol Data Unit is a layer 2 frame that switches use to exchange information about the Spanning Tree Protocol (STP) to prevent network loops.
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