Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
interface GigabitEthernet1/0/12 switchport mode access spanning-tree portfast spanning-tree bpduguard enable
Based on the exhibit, what is the strongest explanation for why the access port shut down immediately after a small unmanaged switch was connected?
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
Best answer
BPDU Guard detected BPDUs on a port expected to face an endpoint.
This is correct because BPDU Guard commonly error-disables such a port when BPDUs appear.
B
Distractor review
Port security rejected the switch because it had a higher bridge ID.
This is wrong because bridge ID is an STP concept, not a port-security check.
C
Distractor review
PPP authentication failed on the access port.
This is wrong because PPP is unrelated to a switched access port receiving BPDUs.
D
Distractor review
DHCP snooping disabled the port because the switch requested an IP address.
This is wrong because the exhibit points directly to BPDU Guard behavior.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is mistaking BPDU Guard’s behavior for port security or DHCP snooping actions. Candidates may incorrectly assume that port security rejects devices based on bridge IDs or that DHCP snooping disables ports when IP requests occur. However, port security controls access by MAC addresses and does not interact with STP bridge IDs, while DHCP snooping focuses on DHCP message validation, not BPDU reception. Misunderstanding these differences leads to selecting incorrect answers that do not explain the immediate shutdown caused by BPDU Guard detecting unexpected BPDUs.
Technical deep dive
How to think about this question
BPDU Guard is a critical Spanning Tree Protocol (STP) security feature designed to protect the network topology from unintended loops and rogue switches. It is typically enabled on access ports that connect end devices, which should not send or forward BPDUs. When a BPDU is received on such a port, BPDU Guard immediately places the port into an error-disabled state, effectively shutting it down to prevent potential Layer 2 loops or topology instability. The decision process behind BPDU Guard is straightforward: access ports are assumed to connect to endpoints, not other switches. If BPDUs appear, it indicates that a switch or device capable of generating BPDUs is connected, violating the expected topology. This triggers BPDU Guard to disable the port, protecting the network from possible broadcast storms or spanning tree recalculations caused by unauthorized devices. A common exam trap is confusing BPDU Guard with other security features like port security or DHCP snooping. Port security focuses on MAC address filtering and does not consider BPDUs or bridge IDs. DHCP snooping protects against rogue DHCP servers but does not react to BPDUs. Understanding BPDU Guard’s role in STP and its behavior in Cisco switches helps avoid these misconceptions and explains why the port shuts down immediately when a small unmanaged switch is connected to an access port with BPDU Guard enabled.
KKey Concepts to Remember
- BPDU Guard is a Cisco feature that disables a port immediately when it receives Bridge Protocol Data Units (BPDUs) on a port configured as an access port facing an endpoint device.
- Access ports are typically configured to connect end devices and should not receive BPDUs, which are used by switches to exchange Spanning Tree Protocol (STP) information.
- When BPDU Guard detects BPDUs on an access port, it error-disables the port to prevent potential Layer 2 loops caused by unauthorized switches or devices.
- Port security does not use bridge IDs for rejecting devices; it controls access based on MAC addresses and violation modes, unrelated to STP bridge IDs.
- PPP authentication is a Layer 2 protocol used on point-to-point serial links and does not apply to Ethernet access ports or BPDU processing.
- DHCP snooping protects against rogue DHCP servers but does not disable ports based on BPDU reception; it focuses on DHCP message validation.
- Spanning Tree Protocol uses BPDUs to detect loops and elect a root bridge, but BPDU Guard enforces topology stability by shutting down ports that receive unexpected BPDUs.
- Error-disabling a port via BPDU Guard is a proactive security measure to maintain network stability and prevent unauthorized switches from causing 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.
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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 port immediately when it receives Bridge Protocol Data Units (BPDUs) on a port configured as an access port facing an endpoint device.
What is the correct answer to this question?
The correct answer is: BPDU Guard detected BPDUs on a port expected to face an endpoint. — The strongest explanation is BPDU Guard on an edge-style port. In practical terms, the port was expected to face an endpoint, not another switch. When BPDUs appeared, the switch treated that as a topology-policy violation and error-disabled the port to protect the network from unintended switching loops or rogue switching behavior. This is an exam-classic access-layer protection pattern and very realistic operationally.
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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