Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit
SW1# show errdisable recovery ErrDisable Reason Timer Status bpduguard Enabled SW1# show interface status err-disabled Port Name Status Reason Gi1/0/11 err-disabled bpduguard
Exhibit: An access switch port shuts down as soon as a user connects a small unmanaged switch under the desk. Which feature caused that behavior?
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
Root Guard
Root Guard blocks a port from becoming root-inconsistent but does not typically err-disable an access edge port on BPDU reception.
B
Best answer
BPDU Guard
BPDU Guard shuts an edge port down if it receives a BPDU.
C
Distractor review
Loop Guard
Loop Guard protects against missing BPDUs on non-designated ports.
D
Distractor review
Storm control
Storm control reacts to traffic thresholds, not BPDUs.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is mistaking BPDU Guard for Root Guard or Loop Guard. Candidates often confuse Root Guard’s role in blocking superior BPDUs with BPDU Guard’s immediate port shutdown on any BPDU reception. Similarly, Loop Guard protects against missing BPDUs but does not err-disable ports. This confusion leads to selecting incorrect answers that do not cause the port to shut down when a small unmanaged switch is connected. Remember, only BPDU Guard err-disables the port upon receiving BPDUs on an access port, which is the exact behavior described in the question.
Technical deep dive
How to think about this question
BPDU Guard is a Cisco feature designed to protect the Spanning Tree Protocol (STP) topology by disabling ports that receive BPDUs unexpectedly. It is typically enabled on access ports that connect to end devices, which should never send BPDUs. When a BPDU is received on such a port, BPDU Guard immediately places the port into an err-disabled state, effectively shutting it down to prevent potential network loops or topology changes caused by unauthorized switches. The decision process behind BPDU Guard is straightforward: if a port configured as an edge port (portfast enabled) receives a BPDU, it is considered a sign that a switch or bridge is connected where only a host should be. This triggers the err-disable action to protect the network. Other STP features like Root Guard and Loop Guard serve different purposes; Root Guard blocks ports from becoming root ports but does not shut them down, while Loop Guard prevents loops caused by missing BPDUs but also does not err-disable ports. A common exam trap is confusing BPDU Guard with Root Guard or Loop Guard. Root Guard blocks superior BPDUs to maintain root bridge stability but does not disable ports. Loop Guard prevents loops from unidirectional link failures but does not shut down ports on BPDU reception. Storm Control is unrelated to BPDUs and focuses on traffic thresholds. Understanding that BPDU Guard specifically protects edge ports by disabling them upon BPDU receipt is critical for correctly answering questions about port shutdowns caused by connecting unmanaged switches.
KKey Concepts to Remember
- BPDU Guard immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.
- Root Guard prevents a port from becoming a root port by blocking superior BPDUs but does not err-disable the port upon BPDU reception.
- Loop Guard protects against unidirectional link failures by preventing a port from transitioning to the forwarding state when BPDUs are missing.
- Storm Control monitors traffic levels on a port and blocks traffic if thresholds are exceeded, but it does not react to BPDU frames.
- Access ports are typically configured with BPDU Guard to protect the network from unauthorized switches that could cause spanning-tree topology changes.
- BPDU Guard helps maintain network stability by err-disabling ports that receive unexpected BPDUs, indicating a potential switch connection where only end devices should be.
- Spanning Tree Protocol (STP) uses BPDUs to detect loops and elect root bridges, and BPDU Guard enforces strict edge port behavior by shutting down ports receiving BPDUs.
- Misconfiguration or unauthorized connection of unmanaged switches to access ports can trigger BPDU Guard, causing the port to shut down to prevent topology changes.
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.
Related practice questions
Related 200-301 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
CCNA subnetting practice questions
Practise IPv4 subnetting, CIDR, masks, host ranges and subnet selection.
CCNA OSPF practice questions
Practise OSPF neighbours, router IDs, metrics, areas and routing-table interpretation.
CCNA VLAN practice questions
Practise VLANs, access ports, trunks, allowed VLANs and switching scenarios.
CCNA STP practice questions
Practise spanning tree, root bridge election, port roles and STP troubleshooting.
CCNA EtherChannel practice questions
Practise LACP, PAgP, port-channel behaviour and bundle requirements.
CCNA ACL practice questions
Practise standard and extended ACLs, permit/deny logic and traffic filtering.
CCNA NAT practice questions
Practise static NAT, dynamic NAT, PAT and inside/outside address translation.
CCNA DHCP practice questions
Practise DHCP scopes, relay, leases and troubleshooting.
CCNA show ip route practice questions
Practise routing-table output, longest-prefix match, AD and route selection.
CCNA show interfaces trunk practice questions
Practise trunk verification and VLAN forwarding across switches.
CCNA wireless security practice questions
Practise WLAN security, authentication and wireless architecture concepts.
CCNA IPv6 practice questions
Practise IPv6 addressing, routes, neighbour discovery and common IPv6 exam traps.
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 immediately disables a switch port if it receives a Bridge Protocol Data Unit (BPDU) on an edge port configured for host devices.
What is the correct answer to this question?
The correct answer is: BPDU Guard — BPDU Guard is meant to protect access ports by shutting them down if BPDUs are received. That usually means someone connected another switch where only an endpoint should exist.
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.
Discussion
Loading comments…
Sign in to join the discussion.