Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Exhibit: A user reports intermittent connectivity after a new switch was connected to an access port. Which feature would have prevented this by immediately disabling the port when a BPDU was received?
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 stops a port from becoming a root port, but it does not simply err-disable any PortFast edge port on BPDU reception.
B
Distractor review
Loop Guard
Loop Guard protects against unidirectional link issues on non-designated ports.
C
Best answer
BPDU Guard
BPDU Guard is the standard protection for PortFast access ports.
D
Distractor review
UDLD
UDLD checks fiber or point-to-point link directionality, not BPDUs from an unexpected switch.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is selecting Root Guard or Loop Guard instead of BPDU Guard. Root Guard prevents a port from becoming a root port by blocking superior BPDUs but does not disable the port upon receiving any BPDU. Loop Guard protects against unidirectional link failures on non-designated ports but does not err-disable ports on BPDU reception. Candidates may incorrectly assume these features provide immediate port shutdown on BPDU receipt, but only BPDU Guard enforces this behavior on PortFast-enabled access ports to prevent bridging loops.
Technical deep dive
How to think about this question
BPDU Guard is a Cisco Spanning Tree Protocol (STP) feature designed to protect access ports configured with PortFast. PortFast allows ports connected to end devices to bypass the usual STP listening and learning states, enabling faster network connectivity. However, if a switch or bridge is accidentally connected to a PortFast-enabled port, it can send Bridge Protocol Data Units (BPDUs), which may cause Layer 2 loops and network instability. BPDU Guard mitigates this risk by immediately err-disabling the port upon receipt of any BPDU, effectively shutting down the port to prevent loops. The decision process behind BPDU Guard is straightforward: when a PortFast-enabled port receives a BPDU, it assumes a misconfiguration or unauthorized device connection. Unlike Root Guard, which blocks ports from becoming root ports but keeps them operational, BPDU Guard takes a more aggressive approach by disabling the port entirely. Loop Guard, on the other hand, protects against unidirectional link failures on non-designated ports but does not err-disable ports on BPDU reception. UDLD focuses on physical link integrity rather than STP BPDU handling. A common exam trap is confusing BPDU Guard with Root Guard or Loop Guard. Root Guard blocks superior BPDUs to protect the root bridge but does not shut down the port. Loop Guard prevents STP topology changes due to unidirectional link failures but does not disable ports on BPDU receipt. BPDU Guard’s unique behavior of err-disabling a PortFast port on BPDU receipt is critical for preventing accidental bridging loops in access layer networks. Practically, network administrators must monitor err-disabled ports and configure errdisable recovery to restore connectivity after resolving the issue.
KKey Concepts to Remember
- BPDU Guard immediately disables a PortFast-enabled access port upon receiving a BPDU to prevent Layer 2 loops caused by unauthorized switches.
- Root Guard prevents a port from becoming a root port by blocking superior BPDUs but does not err-disable the port on BPDU reception.
- Loop Guard protects non-designated STP ports from transitioning to the forwarding state due to unidirectional link failures but does not disable ports on BPDU receipt.
- UDLD detects unidirectional link failures on fiber or point-to-point links by verifying bidirectional communication, unrelated to BPDU handling.
- PortFast is configured on access ports connected to end devices to speed up STP convergence but requires BPDU Guard to protect against accidental switch connections.
- BPDU Guard is essential in environments using PortFast to immediately err-disable ports that receive unexpected BPDUs, preventing network loops.
- A port err-disabled by BPDU Guard requires manual intervention or configured errdisable recovery to re-enable the port after a BPDU violation.
- Cisco switches use BPDU Guard as a proactive STP security feature to maintain network stability by isolating ports that could cause bridging loops.
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 PortFast-enabled access port upon receiving a BPDU to prevent Layer 2 loops caused by unauthorized switches.
What is the correct answer to this question?
The correct answer is: BPDU Guard — BPDU Guard protects PortFast edge ports. If a BPDU appears where an end host is expected, the switch shuts the port down to prevent accidental loops.
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.