CCNA Switching and Network Access Practice Question
A switchport connected to another switch should carry VLANs 10, 20, and 30. The interface is operational, but only VLAN 10 works. VLANs 20 and 30 fail. Which explanation is most likely if the port was accidentally configured as an access port in VLAN 10?
⚠ Common exam trap
Be careful not to confuse access port limitations with trunk configuration issues. Always verify the port mode when troubleshooting VLAN connectivity.
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
✓
The interface is carrying only VLAN 10 because an access port does not transport multiple VLANs like a trunk.
An access port is limited to a single VLAN, so only VLAN 10 traverses the link. Option B is incorrect: IP subnet masks are irrelevant on switchports that operate at Layer 2. Option C is incorrect: inter-switch links typically use trunk ports, not routed ports. Option D is incorrect: STP does not block based on VLAN IDs; it blocks redundant paths, not specific VLANs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The interface is carrying only VLAN 10 because an access port does not transport multiple VLANs like a trunk.
Why this is correct
An access port is configured to carry exactly one VLAN and frames on that link are untagged. A switch-to-switch link that needs to carry VLANs 10, 20, and 30 must be configured as a trunk using 802.1Q tagging, which preserves VLAN membership by inserting a VLAN ID header. Because the port is set to access mode in VLAN 10, it drops or ignores frames from other VLANs, explaining why only VLAN 10 works.
- ✗
VLAN 20 and 30 require different IP subnet masks on the switches.
Why it's wrong here
IP subnet masks apply to Layer 3 addressing, and they have no role in determining which VLANs a switchport carries or forwards. VLANs 20 and 30 could use different IP subnets on their SVIs or router interfaces, but this does not affect the Layer 2 behavior of the switch-to-switch link. The failure to carry VLANs 20 and 30 is purely a configuration issue on the local switchport: it is set as an access port in VLAN 10 instead of a trunk that permits all required VLANs.
When this WOULD be correct
In a different scenario, if the question stated that VLANs 20 and 30 were configured with different subnet masks that did not match the corresponding VLAN interfaces on the switches, then this option would be correct. This would imply that the mismatched subnet masks are preventing proper communication for those VLANs.
- ✗
Every inter-switch link must use a routed port instead of a trunk.
Why it's wrong here
Routed ports do not carry multiple VLANs; they terminate Layer 3 interfaces and forward packets based on IP addresses, not VLANs. Inter-switch links are designed to carry multiple VLANs using a trunk port, which uses 802.1Q tagging to distinguish frames from different VLANs. If you used a routed port, you would need a separate physical link or subinterface for each VLAN, which is impractical and not the standard way to interconnect switches.
When this WOULD be correct
In a question where the requirement specifies that all inter-switch links must operate at Layer 3, and the context indicates that routed ports are necessary for communication between VLANs, this option would be correct. For example, if the question stated that the switches are configured for routing between VLANs instead of switching.
- ✗
STP blocks all VLANs except VLAN 10 by design.
Why it's wrong here
Spanning Tree Protocol (STP) prevents Layer 2 loops by blocking redundant ports based on a port priority or cost per VLAN (or per instance in RSTP/MSTP). It does not arbitrarily block all VLANs except VLAN 10; instead, it can block the same port for one VLAN while forwarding for another, depending on the root bridge per VLAN. Since VLAN 10 is the only one working, the cause is not STP but the access-port configuration, which restricts the link to one untagged VLAN.
When this WOULD be correct
In a different scenario where a question states that STP is configured to block all VLANs except for a designated VLAN due to specific network policies, option D would be correct. For example, if a network engineer intentionally configured STP to prioritize VLAN 10 for critical traffic while blocking others for security reasons.
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.
✓The interface is carrying only VLAN 10 because an access port does not transport multiple VLANs like a trunk.Correct answer▾
Why this is correct
An access port is configured to carry exactly one VLAN and frames on that link are untagged. A switch-to-switch link that needs to carry VLANs 10, 20, and 30 must be configured as a trunk using 802.1Q tagging, which preserves VLAN membership by inserting a VLAN ID header. Because the port is set to access mode in VLAN 10, it drops or ignores frames from other VLANs, explaining why only VLAN 10 works.
✗VLAN 20 and 30 require different IP subnet masks on the switches.Wrong answer — click to see why▾
Why this is wrong here
IP subnet masks are Layer 3 concepts and do not affect Layer 2 VLAN propagation across a switchport. The issue here is purely Layer 2, related to the switchport mode (access vs. trunk), not IP addressing. VLANs 20 and 30 would still fail regardless of subnet mask configuration.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that VLANs 20 and 30 were configured with different subnet masks that did not match the corresponding VLAN interfaces on the switches, then this option would be correct. This would imply that the mismatched subnet masks are preventing proper communication for those VLANs.
Why candidates choose this
Students might confuse VLAN configuration with IP subnetting, thinking that different VLANs require different subnet masks on the switches. However, VLANs are Layer 2 constructs and do not depend on IP subnet masks for their operation.
✗Every inter-switch link must use a routed port instead of a trunk.Wrong answer — click to see why▾
Why this is wrong here
Routed ports are Layer 3 interfaces used for routing between networks, not for carrying multiple VLANs. The standard method for carrying multiple VLANs between switches is to use a trunk port, which tags frames with VLAN IDs. A routed port would not solve the issue and would break Layer 2 connectivity.
★ When this WOULD be the correct answer
In a question where the requirement specifies that all inter-switch links must operate at Layer 3, and the context indicates that routed ports are necessary for communication between VLANs, this option would be correct. For example, if the question stated that the switches are configured for routing between VLANs instead of switching.
Why candidates choose this
Some students may think that routed ports are more advanced or flexible, but they are not designed for VLAN trunking. The confusion arises from the term 'routed' versus 'switched' ports, but trunks are the correct mechanism for inter-switch VLAN transport.
✗STP blocks all VLANs except VLAN 10 by design.Wrong answer — click to see why▾
Why this is wrong here
STP (Spanning Tree Protocol) prevents loops by blocking redundant paths, but it does not selectively block specific VLANs based on their VLAN ID. If STP were blocking VLANs 20 and 30, it would be due to a misconfiguration like PVST+ inconsistencies, not by design. The scenario describes a simple access port misconfiguration, not an STP issue.
★ When this WOULD be the correct answer
In a different scenario where a question states that STP is configured to block all VLANs except for a designated VLAN due to specific network policies, option D would be correct. For example, if a network engineer intentionally configured STP to prioritize VLAN 10 for critical traffic while blocking others for security reasons.
Why candidates choose this
Students might associate STP with VLAN blocking, especially in the context of Per-VLAN Spanning Tree (PVST+), where STP runs per VLAN. However, STP does not inherently block all VLANs except VLAN 10; it would only block specific ports to prevent loops, not arbitrarily drop VLANs.
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?”
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Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
STP
STP (Spanning Tree Protocol) is a network protocol that prevents loops in Ethernet networks by creating a loop-free logical topology.
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.
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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.