The answer is to enable IP routing globally with the `ip routing` command and bring up the main interface GigabitEthernet0/0 using `no shutdown`. This is correct because router-on-a-stick inter-VLAN routing requires both a functional trunk link on the physical interface and a routing process to forward packets between VLANs; without `ip routing`, the router cannot perform inter-VLAN forwarding, and without `no shutdown` on G0/0, the subinterfaces remain administratively down regardless of their correct VLAN encapsulation and IP addresses. On the CCNA 200-301 v2 exam, this scenario tests your understanding that subinterfaces inherit the operational state of the parent interface and that global routing is not enabled by default—a common trap is assuming correct subinterface configuration alone guarantees connectivity. Remember the mnemonic: “Parent must be up, and the brain must route.”
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 connected to R1. Configure router-on-a-stick inter-VLAN routing so that hosts in VLAN 10 and VLAN 20 can communicate through R1. The switch (not shown) is already configured with the correct VLANs and trunk. Troubleshoot and fix any issues in the current R1 configuration.
R1#show running-config | section interface
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 GigabitEthernet0/1
ip address 203.0.113.1 255.255.255.252
duplex auto
speed auto
!
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 203.0.113.0/30 is directly connected, GigabitEthernet0/1
L 203.0.113.1/32 is directly connected, GigabitEthernet0/1
A
Enable IP routing globally and bring up the main interface GigabitEthernet0/0 with 'no shutdown'.
This is correct because inter-VLAN routing on a router-on-a-stick requires IP routing to be enabled globally with 'ip routing' and the main interface (G0/0) must be administratively up. Without these, the router cannot forward packets between VLANs even if subinterfaces are configured correctly.
B
Change the encapsulation on the subinterfaces to 'encapsulation dot1Q 10 native' and 'encapsulation dot1Q 20 native'.
Why wrong: This is incorrect because the 'native' keyword is used only for the native VLAN on a trunk, not for all VLANs. Using 'native' on non-native VLANs would cause mismatched encapsulation and break connectivity.
C
Remove the subinterfaces and configure IP addresses directly on GigabitEthernet0/0.
Why wrong: This is incorrect because configuring an IP address directly on the physical interface does not allow for VLAN tagging. The router would not be able to distinguish between VLAN 10 and VLAN 20 traffic, breaking inter-VLAN routing.
D
Add 'ip routing' on each subinterface individually.
Why wrong: This is incorrect because 'ip routing' is a global command, not an interface-level command. It cannot be applied to subinterfaces; it must be enabled globally on the router.
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 globally and bring up the main interface GigabitEthernet0/0 with 'no shutdown'.
The R1 configuration has two subinterfaces (G0/0.10 and G0/0.20) with correct VLAN encapsulation and IP addresses, but inter-VLAN routing fails because the main interface G0/0 is not configured as a trunk (no 'no shutdown' and no 'ip routing' globally). Enable IP routing globally with 'ip routing' and ensure GigabitEthernet0/0 is administratively up with 'no shutdown'. The subinterfaces will then route between VLANs.
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 globally and bring up the main interface GigabitEthernet0/0 with 'no shutdown'.
Why this is correct
This is correct because inter-VLAN routing on a router-on-a-stick requires IP routing to be enabled globally with 'ip routing' and the main interface (G0/0) must be administratively up. Without these, the router cannot forward packets between VLANs even if subinterfaces are configured correctly.
Related concept
Access ports place end devices into a single VLAN.
✗
Change the encapsulation on the subinterfaces to 'encapsulation dot1Q 10 native' and 'encapsulation dot1Q 20 native'.
Why it's wrong here
This is incorrect because the 'native' keyword is used only for the native VLAN on a trunk, not for all VLANs. Using 'native' on non-native VLANs would cause mismatched encapsulation and break connectivity.
✗
Remove the subinterfaces and configure IP addresses directly on GigabitEthernet0/0.
Why it's wrong here
This is incorrect because configuring an IP address directly on the physical interface does not allow for VLAN tagging. The router would not be able to distinguish between VLAN 10 and VLAN 20 traffic, breaking inter-VLAN routing.
✗
Add 'ip routing' on each subinterface individually.
Why it's wrong here
This is incorrect because 'ip routing' is a global command, not an interface-level command. It cannot be applied to subinterfaces; it must be enabled globally on 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.
✓Enable IP routing globally and bring up the main interface GigabitEthernet0/0 with 'no shutdown'.Correct answer▾
Why this is correct
This is correct because inter-VLAN routing on a router-on-a-stick requires IP routing to be enabled globally with 'ip routing' and the main interface (G0/0) must be administratively up. Without these, the router cannot forward packets between VLANs even if subinterfaces are configured correctly.
✗Change the encapsulation on the subinterfaces to 'encapsulation dot1Q 10 native' and 'encapsulation dot1Q 20 native'.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: The 'native' keyword should only be applied to the subinterface that matches the native VLAN (usually VLAN 1 by default), not to all subinterfaces.
Why candidates choose this
Candidates might think that adding 'native' ensures proper trunking, but it actually misconfigures the encapsulation for non-native VLANs.
✗Remove the subinterfaces and configure IP addresses directly on GigabitEthernet0/0.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: Router-on-a-stick requires subinterfaces with VLAN encapsulation to handle multiple VLANs over a single trunk link. Assigning an IP to the physical interface only works for a single VLAN (usually the native VLAN).
Why candidates choose this
Candidates might think that simplifying the configuration by using the main interface will work, but they overlook the need for VLAN tagging.
✗Add 'ip routing' on each subinterface individually.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: 'ip routing' is a global configuration command that enables Layer 3 forwarding on the entire router. It is not available under interface configuration mode.
Why candidates choose this
Candidates might think that routing needs to be enabled per interface, similar to 'no shutdown', but 'ip routing' is a global setting.
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
Keyword trap
This is incorrect because the 'native' keyword is used only for the native VLAN on a trunk, not for all VLANs. Using 'native' on non-native VLANs would cause mismatched encapsulation and break connectivity.
Command / output trap
This is incorrect because 'ip routing' is a global command, not an interface-level command. It cannot be applied to subinterfaces; it must be enabled globally on the router.
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 globally and bring up the main interface GigabitEthernet0/0 with 'no shutdown'. — The R1 configuration has two subinterfaces (G0/0.10 and G0/0.20) with correct VLAN encapsulation and IP addresses, but inter-VLAN routing fails because the main interface G0/0 is not configured as a trunk (no 'no shutdown' and no 'ip routing' globally). Enable IP routing globally with 'ip routing' and ensure GigabitEthernet0/0 is administratively up with 'no shutdown'. The subinterfaces will then route between VLANs.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You are connected to R1. Configure inter-VLAN routing on R1 using router-on-a-stick so that hosts in VLAN 10 (192.168.10.0/24) and VLAN 20 (192.168.20.0/24) can communicate. The switch SW1 is already configured with VLANs and trunking, but R1's current configuration prevents traffic. Identify and fix the issues.
hard
✓ A.Enable the physical interface with the 'no shutdown' command on R1.
B.Change the encapsulation on the subinterfaces to use dot1Q with native VLAN 10 and 20 respectively.
C.Enable IP routing globally with the 'ip routing' command on R1.
D.Remove the 'no shutdown' from the subinterfaces and apply it only to the physical interface.
Why A: The issue was that the physical interface GigabitEthernet0/0 on R1 was administratively down, causing all subinterfaces for VLANs 10 and 20 to be in a down state. Enabling it with 'no shutdown' brings the trunk up, allowing inter-VLAN routing because the switch already has trunking configured. The other options are incorrect because they suggest steps that are either already in place (IP routing) or not needed (changing encapsulation or moving no shutdown to subinterfaces).
Last reviewed: Jun 6, 2026
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