Question 1,110 of 1,389
CCNA Switching and Network Access Practice Question
Exhibit
MLS1# show running-config | include interface|spanning-tree|portfast|rootguard|loopguard|bpduguard interface GigabitEthernet0/1 spanning-tree guard root interface GigabitEthernet0/2 spanning-tree guard loop interface GigabitEthernet0/3 spanning-tree portfast spanning-tree bpduguard enable ! Current state: MLS1# show spanning-tree interface gigabitEthernet 0/1 detail Port 1 (GigabitEthernet0/1) of VLAN0001 is root inconsistent (Root Guard) Designated bridge has priority 4096, address 0050.7966.6800 Configured guard type: root Root guard active MLS1# show spanning-tree interface gigabitEthernet 0/3 detail Port 3 (GigabitEthernet0/3) of VLAN0001 is down (err-disabled) BPDU guard enabled BPDU received, moving to errdisable state
You are connected to a multilayer switch MLS1. Configure Root Guard on switchport GigabitEthernet 0/1 (connected to an unauthorized switch) so that if a superior BPDU is received, the port is blocked instead of causing a topology change. Also enable Loop Guard on uplink GigabitEthernet 0/2 (connected to the root bridge) to prevent unidirectional link issues. Finally, enable BPDU Guard on PortFast-enabled access port GigabitEthernet 0/3 (connected to a host) so that if a BPDU is received, the port goes err-disabled. After configuration, a superior BPDU is received on G0/1 and the port is blocked; a BPDU is received on G0/3 and the port goes err-disabled. Verify these protections are active.
⚠ Common exam trap
Trap: Mixing up which protection goes where. Root Guard is for ports that should never become root (e.g., facing unauthorized switches). Loop Guard is for ports that are alternate or root ports (uplinks). BPDU Guard is for PortFast-enabled access ports. Also, remember that BPDU Guard triggers err-disable, while Root Guard triggers root-inconsistent (blocking) state.
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
✓
Root Guard on G0/1, Loop Guard on G0/2, BPDU Guard on G0/3
Root Guard was correctly configured on G0/1, so when a superior BPDU arrived, the port entered root-inconsistent state instead of becoming root port. Loop Guard on G0/2 prevents alternate port from becoming root if BPDUs stop. BPDU Guard on G0/3 correctly triggered err-disable upon receiving a BPDU on a PortFast port. To restore G0/3, use 'shutdown' then 'no shutdown' after removing the BPDU source. Verification commands confirm the protections are working.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Root Guard on G0/1, Loop Guard on G0/2, BPDU Guard on G0/3
Why this is correct
Root Guard on G0/1 blocks the port when a superior BPDU is received, placing it in root-inconsistent state. Loop Guard on G0/2 prevents an alternate port from becoming root if BPDUs stop due to unidirectional link. BPDU Guard on G0/3 err-disables the port upon receiving any BPDU, which is correct for a PortFast-enabled access port.
- ✗
Root Guard on G0/1, UplinkFast on G0/2, BPDU Guard on G0/3
Why it's wrong here
UplinkFast is a legacy Cisco proprietary convergence feature that immediately moves a blocked alternate port to forwarding after a root port failure; it does not monitor BPDU continuity and cannot detect a unidirectional link. On G0/2, the correct protection is Loop Guard, which keeps the alternate port in a blocked state when BPDUs stop arriving and places it in loop-inconsistent state rather than allowing it to become root. Therefore the combination fails to address the unidirectional link scenario that the question requires.
- ✗
BPDU Guard on G0/1, Loop Guard on G0/2, Root Guard on G0/3
Why it's wrong here
BPDU Guard on an uplink port (G0/1) would err-disable the port upon receiving a BPDU, which is not desired for a trunk port. Root Guard on an access port (G0/3) would not trigger err-disable; it would only block the port if a superior BPDU is received, but the requirement is to err-disable the port.
- ✗
Root Guard on G0/1, Loop Guard on G0/2, PortFast on G0/3
Why it's wrong here
PortFast is an access-port optimization that changes the initial spanning-tree state machine to start in forwarding, but it offers no protection against unexpected BPDUs and never disables the port. A switch connected to G0/3 could inject BPDUs and create a bridging loop because the port remains in forwarding; BPDU Guard is the feature that err-disables the port upon receiving any BPDU, which is the required behavior. Thus using PortFast alone leaves the access port vulnerable.
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.
✓Root Guard on G0/1, Loop Guard on G0/2, BPDU Guard on G0/3Correct answer▾
Why this is correct
Root Guard on G0/1 blocks the port when a superior BPDU is received, placing it in root-inconsistent state. Loop Guard on G0/2 prevents an alternate port from becoming root if BPDUs stop due to unidirectional link. BPDU Guard on G0/3 err-disables the port upon receiving any BPDU, which is correct for a PortFast-enabled access port.
✗Root Guard on G0/1, UplinkFast on G0/2, BPDU Guard on G0/3Wrong answer — click to see why▾
Why this is wrong here
UplinkFast is not designed to detect or prevent unidirectional links; it only accelerates failover.
Why candidates choose this
Candidates may confuse UplinkFast with Loop Guard because both are STP enhancement features, but they serve different purposes.
✗BPDU Guard on G0/1, Loop Guard on G0/2, Root Guard on G0/3Wrong answer — click to see why▾
Why this is wrong here
The protections are applied to the wrong ports: BPDU Guard should be on access ports, Root Guard on ports facing potential rogue switches, and Loop Guard on uplinks.
Why candidates choose this
Candidates may know the features but misapply them to the wrong interfaces due to confusion about where each protection is appropriate.
✗Root Guard on G0/1, Loop Guard on G0/2, PortFast on G0/3Wrong answer — click to see why▾
Why this is wrong here
PortFast does not prevent BPDU reception; it only skips the listening and learning states. Without BPDU Guard, the port would still process BPDUs and could become a root port.
Why candidates choose this
Candidates may think PortFast is sufficient for access ports, but it must be paired with BPDU Guard to provide security against rogue switches.
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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Last reviewed: Jun 6, 2026
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