The answer is to configure R1’s subinterface GigabitEthernet0/0.30 with encapsulation dot1Q 99 native and add VLAN 30 to SW1’s trunk allowed list. This corrects the native VLAN mismatch because SW1 expects native VLAN 99 on the trunk, but R1’s physical interface defaults to VLAN 1; by setting the subinterface to native 99, both sides agree on untagged traffic. Additionally, VLAN 30 must be explicitly permitted on the trunk since it was omitted, blocking all traffic for that VLAN. On the CCNA 200-301 v2 exam, router-on-a-stick inter-VLAN routing troubleshooting tests your ability to spot mismatched native VLANs and restricted trunk allowed lists—common traps where candidates forget that a subinterface must match the switch’s native VLAN or that VLANs are denied by default. A solid memory tip: “Native must match, allowed must include”—always verify both the native VLAN number and the trunk’s allowed VLAN list when inter-VLAN routing fails.
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 connected to R1. Configure R1 and SW1 so that hosts in VLAN 10 (192.168.10.0/24) and VLAN 20 (192.168.20.0/24) can communicate via the router-on-a-stick setup. The current configuration has errors: the trunk port between SW1 and R1 has a native VLAN mismatch, VLAN 30 is not allowed on the trunk, and the subinterface encapsulation is incorrect. Correct these issues and enable inter-VLAN routing.
SW1#show running-config | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
switchport mode trunk
switchport trunk native vlan 99
switchport trunk allowed vlan 10,20
!
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 30
ip address 192.168.30.1 255.255.255.0
!
R1#show ip route
Codes: C - connected, S - static, I - IGRP, 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
192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.10.0/24 is directly connected, GigabitEthernet0/0.10
L 192.168.10.1/32 is directly connected, GigabitEthernet0/0.10
192.168.20.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.20.0/24 is directly connected, GigabitEthernet0/0.20
L 192.168.20.1/32 is directly connected, GigabitEthernet0/0.20
192.168.30.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.30.0/24 is directly connected, GigabitEthernet0/0.30
L 192.168.30.1/32 is directly connected, GigabitEthernet0/0.30
A
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 99 native; on SW1, allow VLAN 30 on the trunk interface.
This solves the native VLAN mismatch by aligning R1's native VLAN to 99 and ensures VLAN 30 is permitted on the trunk, fulfilling the requirements.
B
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, remove VLAN 30 from the trunk allowed list.
Why wrong: This is incorrect because removing VLAN 30 from the trunk would prevent traffic from that VLAN from reaching the router, breaking inter-VLAN routing.
C
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, set the native VLAN to 1 on the trunk.
Why wrong: This is incorrect because setting the native VLAN to 1 on SW1 would not match R1's subinterface native VLAN 30, causing a mismatch and potential connectivity issues.
D
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, configure the trunk to allow only VLANs 10 and 20.
Why wrong: This is incorrect because VLAN 30 is needed for inter-VLAN routing (as the native VLAN) and must be allowed on the trunk; restricting to VLANs 10 and 20 would block VLAN 30 traffic.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 99 native; on SW1, allow VLAN 30 on the trunk interface.
The native VLAN on SW1 is 99 but R1's physical interface defaults to VLAN 1, causing a mismatch. To fix this, R1's subinterface Gi0/0.30 must be set with encapsulation dot1Q 99 native, making R1's native VLAN 99 and matching SW1. VLAN 30 is not allowed on the trunk, preventing any traffic in that VLAN; it must be added to the trunk's allowed list on SW1. After these corrections, inter-VLAN routing for VLANs 10 and 20 will function correctly.
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.
✓
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 99 native; on SW1, allow VLAN 30 on the trunk interface.
Why this is correct
This solves the native VLAN mismatch by aligning R1's native VLAN to 99 and ensures VLAN 30 is permitted on the trunk, fulfilling the requirements.
Related concept
Access ports place end devices into a single VLAN.
✗
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, remove VLAN 30 from the trunk allowed list.
Why it's wrong here
This is incorrect because removing VLAN 30 from the trunk would prevent traffic from that VLAN from reaching the router, breaking inter-VLAN routing.
✗
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, set the native VLAN to 1 on the trunk.
Why it's wrong here
This is incorrect because setting the native VLAN to 1 on SW1 would not match R1's subinterface native VLAN 30, causing a mismatch and potential connectivity issues.
✗
On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, configure the trunk to allow only VLANs 10 and 20.
Why it's wrong here
This is incorrect because VLAN 30 is needed for inter-VLAN routing (as the native VLAN) and must be allowed on the trunk; restricting to VLANs 10 and 20 would block VLAN 30 traffic.
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.
✓On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 99 native; on SW1, allow VLAN 30 on the trunk interface.Correct answer▾
Why this is correct
This solves the native VLAN mismatch by aligning R1's native VLAN to 99 and ensures VLAN 30 is permitted on the trunk, fulfilling the requirements.
✗On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, remove VLAN 30 from the trunk allowed list.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: VLAN 30 must be allowed on the trunk for the router to receive and forward traffic for that VLAN.
Why candidates choose this
Candidates might think that since VLAN 30 is the native VLAN, it doesn't need to be explicitly allowed, but Cisco trunks require explicit permission.
✗On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, set the native VLAN to 1 on the trunk.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: The native VLAN must match on both ends of the trunk; changing SW1 to VLAN 1 would not resolve the mismatch with R1's native VLAN 30.
Why candidates choose this
Candidates may assume that default native VLAN 1 is always correct, but the question states SW1 uses native VLAN 99 (or 30 in the explanation), so consistency is key.
✗On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 30; on SW1, configure the trunk to allow only VLANs 10 and 20.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: The trunk must allow all VLANs that need to be routed, including the native VLAN (30), otherwise the router cannot communicate with hosts in VLAN 30.
Why candidates choose this
Candidates might think only user VLANs (10 and 20) need to be allowed, forgetting that the native VLAN is also used for management or routing.
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.
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: On R1, configure interface GigabitEthernet0/0.30 with encapsulation dot1Q 99 native; on SW1, allow VLAN 30 on the trunk interface. — The native VLAN on SW1 is 99 but R1's physical interface defaults to VLAN 1, causing a mismatch. To fix this, R1's subinterface Gi0/0.30 must be set with encapsulation dot1Q 99 native, making R1's native VLAN 99 and matching SW1. VLAN 30 is not allowed on the trunk, preventing any traffic in that VLAN; it must be added to the trunk's allowed list on SW1. After these corrections, inter-VLAN routing for VLANs 10 and 20 will function correctly.
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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