The answer is to enable the 'ip routing' command in global configuration mode. This is correct because a router-on-a-stick configuration relies on the router’s ability to forward packets between VLANs, and by default, IP routing is disabled on Cisco IOS devices. Without this command, the router will not perform inter-VLAN routing, even if subinterfaces and encapsulation are perfectly configured. On the CCNA 200-301 v2 exam, this scenario tests your understanding that a router must explicitly be told to route—a common trap is assuming that simply configuring subinterfaces with 'encapsulation dot1q' is enough. A frequent memory tip is to think of 'ip routing' as the master switch that turns a router from a simple host into a Layer 3 forwarding device. If hosts in different VLANs cannot ping each other despite correct subinterface IPs and a 'no shutdown' on the physical port, always check for the missing 'ip routing' command first.
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.
Exhibit
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
!
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
C 192.168.20.0/24 is directly connected, GigabitEthernet0/0.20
You are connected to R1. The network consists of R1, SW1, and two VLANs (10 and 20). SW1 has hosts in VLAN 10 and VLAN 20 connected to its access ports. Configure R1 for router-on-a-stick inter-VLAN routing using subinterfaces on G0/0. The physical interface G0/0 is administratively up (no shutdown). The current configuration is incomplete and has errors preventing communication between the VLANs. Fix the configuration so that hosts in VLAN 10 (192.168.10.0/24) and VLAN 20 (192.168.20.0/24) can ping each other through R1.
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
!
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
C 192.168.20.0/24 is directly connected, GigabitEthernet0/0.20
A
Enable IP routing with the 'ip routing' global configuration command.
The 'ip routing' command enables IPv4 routing on the router. Without it, the router cannot forward packets between subinterfaces, even if they are correctly configured. This is the missing piece that prevents inter-VLAN communication.
B
Change the encapsulation on subinterface G0/0.10 to dot1Q 20 and on G0/0.20 to dot1Q 10.
Why wrong: This is incorrect because the VLAN-to-subinterface mapping is already correct: VLAN 10 on G0/0.10 and VLAN 20 on G0/0.20. Swapping them would cause mismatched VLANs and break connectivity.
C
Add the 'no shutdown' command on the physical interface G0/0.
Why wrong: This is incorrect because the physical interface must be 'no shutdown' for subinterfaces to work, but the question implies the interface is already up. The missing 'ip routing' is the primary issue, not the interface state.
D
Configure the native VLAN on the trunk to match the subinterface for VLAN 1.
Why wrong: This is incorrect because the native VLAN mismatch is not indicated as an issue in the scenario. The primary fault is the missing 'ip routing' command. Native VLAN configuration is important but not the root cause here.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable IP routing with the 'ip routing' global configuration command.
The primary fault is that IP routing is disabled globally on R1 (missing 'ip routing' command), so even though subinterfaces are correctly configured, the router cannot route between VLANs. The physical interface G0/0 is already administratively up, so issuing 'no shutdown' would not fix the problem. Thus, enabling 'ip routing' resolves the inter-VLAN communication issue. The other options either incorrectly swap encapsulation or address a native VLAN scenario not indicated in the exhibit.
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.
✓
Enable IP routing with the 'ip routing' global configuration command.
Why this is correct
The 'ip routing' command enables IPv4 routing on the router. Without it, the router cannot forward packets between subinterfaces, even if they are correctly configured. This is the missing piece that prevents inter-VLAN communication.
Related concept
Access ports place end devices into a single VLAN.
✗
Change the encapsulation on subinterface G0/0.10 to dot1Q 20 and on G0/0.20 to dot1Q 10.
Why it's wrong here
This is incorrect because the VLAN-to-subinterface mapping is already correct: VLAN 10 on G0/0.10 and VLAN 20 on G0/0.20. Swapping them would cause mismatched VLANs and break connectivity.
✗
Add the 'no shutdown' command on the physical interface G0/0.
Why it's wrong here
This is incorrect because the physical interface must be 'no shutdown' for subinterfaces to work, but the question implies the interface is already up. The missing 'ip routing' is the primary issue, not the interface state.
✗
Configure the native VLAN on the trunk to match the subinterface for VLAN 1.
Why it's wrong here
This is incorrect because the native VLAN mismatch is not indicated as an issue in the scenario. The primary fault is the missing 'ip routing' command. Native VLAN configuration is important but not the root cause here.
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.
✓Enable IP routing with the 'ip routing' global configuration command.Correct answer▾
Why this is correct
The 'ip routing' command enables IPv4 routing on the router. Without it, the router cannot forward packets between subinterfaces, even if they are correctly configured. This is the missing piece that prevents inter-VLAN communication.
✗Change the encapsulation on subinterface G0/0.10 to dot1Q 20 and on G0/0.20 to dot1Q 10.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the encapsulation commands are already correctly assigned; swapping them would misconfigure the subinterfaces.
Why candidates choose this
Candidates might think the VLAN IDs are swapped because they misread the configuration or assume a common mistake of reversing the numbers.
✗Add the 'no shutdown' command on the physical interface G0/0.Wrong answer — click to see why▾
Why this is wrong here
The physical interface G0/0 is already administratively up, so 'no shutdown' would have no effect and does not fix the disabled routing process.
Why candidates choose this
Candidates often forget to enable interfaces and assume that is the issue, especially when subinterfaces are configured but the physical interface is down.
✗Configure the native VLAN on the trunk to match the subinterface for VLAN 1.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that native VLAN mismatch would cause specific symptoms (e.g., CDP errors) but is not the problem described; the router cannot route even with correct trunking.
Why candidates choose this
Candidates often focus on trunking issues like native VLAN mismatch because it is a common problem in router-on-a-stick setups, but here the routing process itself is disabled.
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 the native VLAN mismatch is not indicated as an issue in the scenario. The primary fault is the missing 'ip routing' command. Native VLAN configuration is important but not the root cause here.
Scenario analysis trap
This is incorrect because the native VLAN mismatch is not indicated as an issue in the scenario. The primary fault is the missing 'ip routing' command. Native VLAN configuration is important but not the root cause here.
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: Enable IP routing with the 'ip routing' global configuration command. — The primary fault is that IP routing is disabled globally on R1 (missing 'ip routing' command), so even though subinterfaces are correctly configured, the router cannot route between VLANs. The physical interface G0/0 is already administratively up, so issuing 'no shutdown' would not fix the problem. Thus, enabling 'ip routing' resolves the inter-VLAN communication issue. The other options either incorrectly swap encapsulation or address a native VLAN scenario not indicated in the exhibit.
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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