Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit
interface g1/0/24 switchport mode access spanning-tree portfast spanning-tree bpduguard enable
Exhibit: SW1 is the root bridge for VLAN 10. A user switch receives a BPDU on an access port connected to a desk-side unmanaged switch. What should happen if BPDU Guard is enabled on that port?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
The port transitions to forwarding more quickly
That is PortFast behavior, not BPDU Guard after receiving a BPDU.
B
Best answer
The port is moved to err-disabled state
BPDU Guard shuts the port down when a BPDU is seen on an edge port.
C
Distractor review
The switch elects a new root bridge
A BPDU on one edge port does not force a new root election.
D
Distractor review
The port becomes a trunk automatically
STP protections do not convert access ports into trunks.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is mistaking BPDU Guard’s function for PortFast behavior. PortFast allows an access port to transition quickly to forwarding but does not disable the port if a BPDU is received. Candidates might incorrectly select that the port transitions to forwarding faster (Option A), confusing BPDU Guard with PortFast. Another trap is thinking that receiving a BPDU on an edge port triggers a new root bridge election (Option C), but root election depends on the entire network’s BPDU priorities, not a single port’s BPDU reception. Also, some may wrongly believe the port becomes a trunk automatically (Option D), which is not a function of BPDU Guard or STP protections.
Technical deep dive
How to think about this question
Spanning Tree Protocol (STP) uses Bridge Protocol Data Units (BPDUs) to detect loops and maintain a loop-free Layer 2 topology. Edge ports, typically connected to end devices, are configured with PortFast to speed up their transition to forwarding state by bypassing the usual STP listening and learning states. However, if a BPDU is received on such a port, it indicates that another switch or device capable of generating BPDUs is connected, which could cause loops or topology instability. BPDU Guard is a protective feature that disables a PortFast-enabled port immediately upon receipt of a BPDU. This action places the port into an err-disabled state, effectively shutting it down to prevent potential Layer 2 loops or rogue switches from affecting the network. The err-disabled state requires administrative intervention or configured automatic recovery to bring the port back up. This mechanism ensures that only trusted devices connect to edge ports, maintaining the integrity of the STP topology and the root bridge election process. A common exam trap is confusing BPDU Guard’s behavior with PortFast or STP root bridge election. While PortFast speeds up port activation, it does not shut down ports on BPDU receipt. BPDU Guard specifically disables the port to prevent loops. Also, receiving a BPDU on an edge port does not trigger a new root bridge election; root election depends on BPDU priority and path cost across the entire network. Understanding these distinctions is critical for correctly answering questions about STP protections and port security in the CCNA exam.
KKey Concepts to Remember
- BPDU Guard is a Cisco feature that disables a PortFast-enabled access port if it receives a Bridge Protocol Data Unit (BPDU), preventing potential Layer 2 loops.
- When BPDU Guard is enabled on an access port, the switch immediately moves the port to an err-disabled state upon receiving any BPDU frames.
- PortFast allows access ports to transition quickly to the forwarding state, but BPDU Guard protects these ports from connecting unauthorized switches.
- Receiving a BPDU on a PortFast-enabled port indicates a possible misconfiguration or rogue device, triggering BPDU Guard to shut down the port.
- BPDU Guard does not cause the switch to elect a new root bridge; root bridge election is based on BPDU priority and path cost.
- BPDU Guard does not convert access ports into trunk ports; port type remains unchanged regardless of BPDU reception.
- The err-disabled state caused by BPDU Guard requires manual or automatic recovery to re-enable the port after the BPDU violation is resolved.
- BPDU Guard is essential for securing edge ports in VLAN environments, especially in VLAN 10 where STP root bridge stability is critical.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
BPDU Guard is a Cisco feature that disables a PortFast-enabled access port if it receives a Bridge Protocol Data Unit (BPDU), preventing potential Layer 2 loops.
What is the correct answer to this question?
The correct answer is: The port is moved to err-disabled state — BPDU Guard is designed to protect edge ports. If a BPDU is received on a PortFast access port, the switch places the interface into the err-disabled state to stop a potential Layer 2 loop or rogue switch.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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