The answer is to change the encapsulation on subinterface G0/0.30 to dot1Q 30 and reapply the IP address. This resolves the encapsulation dot1q mismatch because the subinterface for VLAN 30 is incorrectly tagging frames with VLAN 100 instead of VLAN 30, preventing the router from forwarding packets to that VLAN. Since VLAN 20 is correctly configured, hosts in VLAN 10 can reach VLAN 20, but the mismatch blocks all traffic to VLAN 30. On the CCNA 200-301 v2 exam, this scenario tests your understanding of router-on-a-stick inter-VLAN routing and the critical role of matching encapsulation VLAN IDs to the switch’s VLAN database. A common trap is assuming IP routing is disabled or that the native VLAN needs a subinterface—neither is true here. Memory tip: “Match the tag, or the packet will lag”—always verify that the encapsulation dot1Q number on each subinterface exactly matches the VLAN ID it serves.
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Network Topology
You are troubleshooting inter-VLAN routing on a router-on-a-stick setup. R1 is connected to SW1 via trunk port G0/0. VLANs 10, 20, and 30 exist on SW1, and R1 should route between them. Currently, hosts in VLAN 10 can communicate with VLAN 20 but cannot reach VLAN 30. Review the provided configuration and fix the issue.
R1#show running-config | section interface GigabitEthernet0/0
interface GigabitEthernet0/0
no ip address
duplex auto
speed auto
!
interface GigabitEthernet0/0.10
encapsulation dot1Q 10
ip address 192.168.10.1 255.255.255.0
!
interface GigabitEthernet0/0.20
encapsulation dot1Q 20
ip address 192.168.20.1 255.255.255.0
!
interface GigabitEthernet0/0.30
encapsulation dot1Q 100
ip address 192.168.30.1 255.255.255.0
!
R1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C 192.168.10.0/24 is directly connected, GigabitEthernet0/0.10
L 192.168.10.1/32 is directly connected, GigabitEthernet0/0.10
C 192.168.20.0/24 is directly connected, GigabitEthernet0/0.20
L 192.168.20.1/32 is directly connected, GigabitEthernet0/0.20
C 192.168.30.0/24 is directly connected, GigabitEthernet0/0.30
L 192.168.30.1/32 is directly connected, GigabitEthernet0/0.30
SW1#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gig0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gig0/1 1-1005
Port Vlans allowed and active in management domain
Gig0/1 1,10,20,30
Port Vlans in spanning tree forwarding state and not pruned
Gig0/1 10,20,30
A
Change the encapsulation on subinterface G0/0.30 to dot1Q 30 and reapply the IP address.
The subinterface G0/0.30 is configured with encapsulation dot1Q 100, which does not match VLAN 30. Changing it to dot1Q 30 ensures the subinterface correctly tags frames for VLAN 30, enabling routing between VLANs.
B
Add a subinterface for native VLAN 1 on R1 and assign an IP address.
Why wrong: This is incorrect because the native VLAN (VLAN 1) does not require a subinterface unless hosts in VLAN 1 need to be routed. The problem is with VLAN 30, not the native VLAN.
C
Enable IP routing on R1 with the 'ip routing' command.
Why wrong: This is incorrect because IP routing is enabled by default on Cisco routers. The command 'ip routing' is not needed and would not fix the VLAN mismatch.
D
Change the allowed VLAN list on the trunk to include VLAN 30.
Why wrong: This is incorrect because the trunk already allows VLANs 10, 20, and 30 as stated in the scenario. The issue is on the router side, not the switch trunk.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Change the encapsulation on subinterface G0/0.30 to dot1Q 30 and reapply the IP address.
The subinterface for VLAN 30 (G0/0.30) is misconfigured with encapsulation dot1Q 100 instead of dot1Q 30, so R1 cannot forward packets to VLAN 30. VLAN 20 is correctly set up, which is why VLAN 10-to-VLAN 20 pings succeed. Changing the encapsulation to dot1Q 30 and reapplying the IP address restores full inter-VLAN routing. IP routing is enabled by default, so no additional command is needed; the native VLAN does not require a subinterface because no hosts are in that VLAN.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Change the encapsulation on subinterface G0/0.30 to dot1Q 30 and reapply the IP address.
Why this is correct
The subinterface G0/0.30 is configured with encapsulation dot1Q 100, which does not match VLAN 30. Changing it to dot1Q 30 ensures the subinterface correctly tags frames for VLAN 30, enabling routing between VLANs.
Related concept
Access ports place end devices into a single VLAN.
✗
Add a subinterface for native VLAN 1 on R1 and assign an IP address.
Why it's wrong here
This is incorrect because the native VLAN (VLAN 1) does not require a subinterface unless hosts in VLAN 1 need to be routed. The problem is with VLAN 30, not the native VLAN.
✗
Enable IP routing on R1 with the 'ip routing' command.
Why it's wrong here
This is incorrect because IP routing is enabled by default on Cisco routers. The command 'ip routing' is not needed and would not fix the VLAN mismatch.
✗
Change the allowed VLAN list on the trunk to include VLAN 30.
Why it's wrong here
This is incorrect because the trunk already allows VLANs 10, 20, and 30 as stated in the scenario. The issue is on the router side, not the switch trunk.
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.
✓Change the encapsulation on subinterface G0/0.30 to dot1Q 30 and reapply the IP address.Correct answer▾
Why this is correct
The subinterface G0/0.30 is configured with encapsulation dot1Q 100, which does not match VLAN 30. Changing it to dot1Q 30 ensures the subinterface correctly tags frames for VLAN 30, enabling routing between VLANs.
✗Add a subinterface for native VLAN 1 on R1 and assign an IP address.Wrong answer — click to see why▾
Why this is wrong here
The native VLAN is untagged on the trunk, and R1 can handle it via the physical interface if needed, but a missing subinterface for native VLAN 1 is not the cause of the issue.
Why candidates choose this
Candidates may think that all VLANs on the trunk must have a corresponding subinterface, including the native VLAN, but this is not required unless routing for that VLAN is needed.
✗Enable IP routing on R1 with the 'ip routing' command.Wrong answer — click to see why▾
Why this is wrong here
IP routing is already enabled; the issue is a configuration error on the subinterface, not a missing global command.
Why candidates choose this
Candidates may confuse router-on-a-stick with a Layer 3 switch, where 'ip routing' might be disabled by default, but on a router it is enabled by default.
✗Change the allowed VLAN list on the trunk to include VLAN 30.Wrong answer — click to see why▾
Why this is wrong here
The trunk configuration is correct; the problem is the encapsulation mismatch on the router subinterface.
Why candidates choose this
Candidates may assume the trunk is blocking VLAN 30, but the scenario explicitly states VLANs 10, 20, and 30 exist on SW1 and the trunk allows them.
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?”
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
This is incorrect because IP routing is enabled by default on Cisco routers. The command 'ip routing' is not needed and would not fix the VLAN mismatch.
Scenario analysis trap
This is incorrect because the trunk already allows VLANs 10, 20, and 30 as stated in the scenario. The issue is on the router side, not the switch trunk.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
Switching and Network Access — This question tests Switching and Network Access — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Change the encapsulation on subinterface G0/0.30 to dot1Q 30 and reapply the IP address. — The subinterface for VLAN 30 (G0/0.30) is misconfigured with encapsulation dot1Q 100 instead of dot1Q 30, so R1 cannot forward packets to VLAN 30. VLAN 20 is correctly set up, which is why VLAN 10-to-VLAN 20 pings succeed. Changing the encapsulation to dot1Q 30 and reapplying the IP address restores full inter-VLAN routing. IP routing is enabled by default, so no additional command is needed; the native VLAN does not require a subinterface because no hosts are in that VLAN.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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