CCNA Switching and Network Access Practice Question
Exhibit
SW1# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 1-4094 Port Vlans allowed and active in management domain Gi0/1 1,10,20 Port Vlans in spanning tree forwarding state and not pruned Gi0/1 1,10,20 SW1# show running-config interface gi0/1 Building configuration... Current configuration : 150 bytes ! interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport mode trunk switchport trunk native vlan 1 end SW1# show vlan brief VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 1 default active Gi0/1, Gi0/2, Gi0/3 10 VLAN0010 active Gi0/4 20 VLAN0020 active Gi0/5 1002 fddi-default act/unsup 1003 token-ring-default act/unsup 1004 fddinet-default act/unsup 1005 trnet-default act/unsup
A network engineer is troubleshooting connectivity between two hosts in different VLANs on the same switch. Host A in VLAN 10 (10.10.10.5/24) cannot ping Host B in VLAN 20 (10.10.20.5/24). The switch is configured as a router-on-a-stick with a trunk port to an external router. The trunk port is up/up, but inter-VLAN routing fails. What is the most likely cause?
⚠ Common exam trap
A common trap is assuming an up/up trunk automatically passes traffic for all VLANs, overlooking that the allowed VLAN list can be manually pruned and must include every VLAN that needs to traverse the trunk.
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 VLANs are not allowed on the trunk; the allowed VLAN list is missing VLAN 10 and 20.
The most likely cause is that the allowed VLAN list on the trunk is missing VLAN 10 and 20. Even though the trunk port is up/up, if the switch's allowed VLAN list has been restricted (for example, using the switchport trunk allowed vlan command) and does not include those VLANs, all frames tagged with VLAN 10 or 20 will be discarded at the trunk. This directly prevents inter-VLAN routing despite the trunk being operational, whereas a native VLAN mismatch only affects untagged traffic and would not impact the tagged frames between the two hosts.
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 trunk port is not in trunking mode; it is in dynamic desirable mode.
Why it's wrong here
The show interfaces trunk output explicitly lists the port with mode 'on' and status 'trunking', proving that the trunk link is operational and not relying on DTP. Even if the port were in dynamic desirable mode, it could still negotiate a trunk with a compatible neighbor, but the output confirms that it is already trunking. Therefore, the failure is not caused by the trunk mode being misconfigured; the allowed VLAN list is the actual culprit.
- ✗
The native VLAN on the switch trunk is VLAN 1, but the router subinterface for VLAN 1 is not configured with the 'native' keyword or is missing.
Why it's wrong here
The switch trunk native VLAN is 1, which is also used for default VLAN. If the router subinterface for VLAN 1 does not have 'encapsulation dot1q 1 native', the router will expect tagged frames for VLAN 1, but the switch sends them untagged. This mismatch causes communication failure for the native VLAN, which may be used for management or routing. The fix is to either change the native VLAN on the switch to an unused VLAN or add the 'native' keyword on the router subinterface.
When this WOULD be correct
This is the root cause because the native VLAN mismatch disrupts the control plane or routing traffic.
- ✓
The VLANs are not allowed on the trunk; the allowed VLAN list is missing VLAN 10 and 20.
Why this is correct
When an allowed VLAN list on a trunk is configured and does not include VLAN 10 and VLAN 20, all tagged frames from those VLANs are dropped, causing inter-VLAN routing failure even though the trunk is up/up.
- ✗
The switch ports Gi0/4 and Gi0/5 are in access mode but not assigned to the correct VLANs.
Why it's wrong here
The 'show vlan brief' output displays Gi0/4 as a member of VLAN 10 and Gi0/5 as a member of VLAN 20, which matches the intended access VLANs for the attached hosts. Since the access ports are correctly assigned, frames from the hosts are being placed into the proper VLANs and forwarded toward the trunk. The issue is not with the access port configuration but with the trunk's allowed VLAN list, which drops VLAN 10 and VLAN 20 traffic before it reaches the router.
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 VLANs are not allowed on the trunk; the allowed VLAN list is missing VLAN 10 and 20.Correct answer▾
Why this is correct
When an allowed VLAN list on a trunk is configured and does not include VLAN 10 and VLAN 20, all tagged frames from those VLANs are dropped, causing inter-VLAN routing failure even though the trunk is up/up.
✗The trunk port is not in trunking mode; it is in dynamic desirable mode.Wrong answer — click to see why▾
Why this is wrong here
The trunk is already operational.
✗The native VLAN on the switch trunk is VLAN 1, but the router subinterface for VLAN 1 is not configured with the 'native' keyword or is missing.Wrong answer — click to see why▾
Why this is wrong here
A native VLAN mismatch only impacts untagged frames; the hosts send tagged traffic, so a missing 'native' keyword on the router subinterface does not block the VLAN 10 and VLAN 20 frames.
★ When this WOULD be the correct answer
This is the root cause because the native VLAN mismatch disrupts the control plane or routing traffic.
✗The switch ports Gi0/4 and Gi0/5 are in access mode but not assigned to the correct VLANs.Wrong answer — click to see why▾
Why this is wrong here
The access ports are properly assigned to the respective 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
Native VLAN mismatch
A Native VLAN mismatch occurs when two connected switches have different Native VLANs configured on the trunk port, causing control traffic like CDP, DTP, and BPDUs to be sent on the wrong VLAN and potentially creating security vulnerabilities or connectivity issues.
Key term
Inter-VLAN routing
Inter-VLAN routing is the process of forwarding network traffic between different VLANs to enable communication across logically separated broadcast domains.
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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.