CCNA Switching and Network Access Practice Question
A switch receives BPDUs on a user-facing port configured as an edge port, but instead of just blocking the port role it fully error-disables it. Which protection feature most likely explains that behavior?
⚠ Common exam trap
Be careful not to confuse BPDU Guard with other spanning tree protection features that do not disable ports upon receiving BPDUs.
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
✓
BPDU Guard
BPDU Guard most likely explains that behavior. In practical terms, BPDU Guard is used to protect ports that are expected to face ordinary endpoints, not other switches. If BPDUs appear on such a port, the device treats that as a serious topology-policy violation and shuts the port down. This is different from features that merely influence spanning-tree role choice without fully error-disabling the interface.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
BPDU Guard
Why this is correct
BPDU Guard is a security feature designed for edge or access ports, typically used with PortFast. When a port configured with BPDU Guard receives any BPDU, it immediately transitions to the err-disabled state, because an unexpected BPDU indicates that a switch or bridge is connected to an end-user segment. This prevents the possibility of a bridging loop caused by a rogue switch and does not require manual intervention beyond the initial configuration, though recovery can be automatic if err-disable timeout is set.
- ✗
Root guard
Why it's wrong here
Root guard is a spanning-tree protection that monitors incoming BPDUs to preserve the position of the root bridge in the topology. If a superior BPDU arrives on a root-guard-enabled port, the port is put into a root-inconsistent state, but it is not error-disabled or shutdown; rather, it is placed in a blocking state that prevents the switch from accepting a new root via that port. This is a role-based protection mechanism, not a direct err-disable response to BPDU reception, making it distinct from BPDU Guard's behavior.
When this WOULD be correct
In a scenario where a switch port is configured to prevent a non-root bridge from becoming the root bridge, a question could ask about the behavior of a port receiving BPDUs on a non-edge port. In that case, root guard would be the correct answer as it would block the port to maintain the intended topology.
- ✗
Port security
Why it's wrong here
Port security is a Layer 2 mechanism that restricts the number of unique source MAC addresses allowed on a port and can define violation actions such as restrict, protect, or shutdown. It operates entirely on MAC address learning behavior and has no capability to inspect or interpret BPDUs. Receiving BPDUs on a port with port security will not trigger a violation unless the MAC address limit is surpassed or an allowed MAC address is violated, so it is not the correct feature for responding to unexpected BPDUs.
When this WOULD be correct
If the question were about a switch port configured with port security that had a violation due to exceeding the allowed number of MAC addresses, and the switch subsequently error-disabled the port, then 'Port security' would be the correct answer.
- ✗
DHCP snooping
Why it's wrong here
DHCP snooping is a Layer 2 security feature that filters DHCP messages based on the trusted or untrusted status of a port, preventing rogue DHCP servers from assigning IP addresses. It has absolutely no interaction with BPDUs, as those are spanning-tree protocol frames handled by the switch's STP engine. A port with DHCP snooping can receive BPDUs without any error-disabling or shutdown occurrence, and DHCP snooping does not even parse BPDU content, so it cannot be the mechanism that disables the port in this scenario.
When this WOULD be correct
In a scenario where the question asks about a switch port that is configured to prevent rogue DHCP servers and is receiving DHCP packets from an unauthorized source, DHCP snooping would be the correct answer if the port was shut down due to this violation.
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.
✓BPDU GuardCorrect answer▾
Why this is correct
BPDU Guard is a security feature designed for edge or access ports, typically used with PortFast. When a port configured with BPDU Guard receives any BPDU, it immediately transitions to the err-disabled state, because an unexpected BPDU indicates that a switch or bridge is connected to an end-user segment. This prevents the possibility of a bridging loop caused by a rogue switch and does not require manual intervention beyond the initial configuration, though recovery can be automatic if err-disable timeout is set.
✗Root guardWrong answer — click to see why▾
Why this is wrong here
Root guard does not error-disable a port; instead, it places the port into a root-inconsistent state if a superior BPDU is received, preventing the port from becoming a root port. It is used to enforce the root bridge location, not to disable ports upon BPDU reception.
★ When this WOULD be the correct answer
In a scenario where a switch port is configured to prevent a non-root bridge from becoming the root bridge, a question could ask about the behavior of a port receiving BPDUs on a non-edge port. In that case, root guard would be the correct answer as it would block the port to maintain the intended topology.
Why candidates choose this
Students may confuse root guard with BPDU Guard because both involve BPDU processing and protection. However, root guard's behavior is to keep the port operational but in a special state, not to error-disable it.
✗Port securityWrong answer — click to see why▾
Why this is wrong here
Port security restricts the number of MAC addresses learned on a port and can error-disable the port if a violation occurs (e.g., too many MAC addresses). It does not react to BPDUs; its focus is on MAC address learning, not spanning-tree BPDUs.
★ When this WOULD be the correct answer
If the question were about a switch port configured with port security that had a violation due to exceeding the allowed number of MAC addresses, and the switch subsequently error-disabled the port, then 'Port security' would be the correct answer.
Why candidates choose this
Port security is a common feature that causes error-disable, so test-takers might assume it applies here. However, the trigger for port security is MAC address violations, not BPDU reception.
✗DHCP snoopingWrong answer — click to see why▾
Why this is wrong here
DHCP snooping is a security feature that filters DHCP messages and can error-disable a port if a DHCP violation occurs (e.g., rogue DHCP server). It does not inspect or react to BPDUs, which are layer 2 spanning-tree frames.
★ When this WOULD be the correct answer
In a scenario where the question asks about a switch port that is configured to prevent rogue DHCP servers and is receiving DHCP packets from an unauthorized source, DHCP snooping would be the correct answer if the port was shut down due to this violation.
Why candidates choose this
DHCP snooping is another feature that can cause error-disable, leading to confusion. But its trigger is DHCP-related, not BPDU-related, so it is not applicable here.
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?”
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
Key term
BPDU Guard
BPDU Guard is a Cisco switch feature that protects the network from unauthorized devices by disabling a port if it receives a Bridge Protocol Data Unit (BPDU).
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.