The answer is to enable the 'ip routing' command in global configuration mode on R1. This command is essential because a Cisco router, by default, operates as a host and will not forward packets between its interfaces or subinterfaces; without it, the router cannot perform the inter-VLAN routing required in a router-on-a-stick design. On the CCNA 200-301 v2 exam, this scenario tests your understanding that a router must have IP routing explicitly enabled to act as a gateway between VLANs, and a common trap is assuming that simply configuring subinterfaces with encapsulation and IP addresses is sufficient. The missing 'ip routing' command is a frequent oversight, especially when transitioning from a Layer 3 switch, which has IP routing enabled by default. Remember the memory tip: "No route, no router"—if you don't tell the router to route, it won't.
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
R1# show interfaces trunk
No trunks exist
R1#
You are connected to R1. The network consists of R1, a router, and SW1, a Layer 2 switch. VLANs 10 (192.168.10.0/24) and 20 (192.168.20.0/24) are configured on SW1 with hosts in each VLAN. R1 must perform inter-VLAN routing using a router-on-a-stick configuration on interface G0/0. Currently, hosts in VLAN 10 cannot ping hosts in VLAN 20. Configure R1 and identify and resolve 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
!
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
R1# show interfaces trunk
No trunks exist
R1#
A
Enable IP routing on R1 with the 'ip routing' global configuration command.
The 'ip routing' command enables IP routing on a Cisco router, which is required for forwarding packets between subinterfaces in a router-on-a-stick configuration. Without it, the router will not route between VLANs.
B
Configure the physical interface G0/0 with an IP address and enable trunking with 'switchport mode trunk'.
Why wrong: In a router-on-a-stick configuration, the physical interface does not have an IP address; IP addresses are assigned to subinterfaces. Also, 'switchport mode trunk' is a switch command, not used on routers.
C
Ensure the trunk between R1 and SW1 is configured with 'switchport trunk allowed vlan 10,20' on SW1.
Why wrong: While this command is important on the switch side to allow VLANs on the trunk, the question focuses on R1's configuration. The issue described (missing 'ip routing') is on the router, not the switch.
D
Add the 'no shutdown' command on R1's subinterfaces G0/0.10 and G0/0.20.
Why wrong: Subinterfaces inherit the physical interface's state; they do not have their own 'shutdown' state. The 'no shutdown' command is only effective on the physical interface.
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 on R1 with the 'ip routing' global configuration command.
The problem is that R1 is missing the 'ip routing' command, which is required to enable IP routing on a router. Without it, R1 cannot forward packets between the subinterfaces. Additionally, the trunk between R1 and SW1 may not be operational because R1's physical interface G0/0 has no IP address and is not set to trunk mode (though subinterfaces handle encapsulation). The solution is to enable IP routing globally and ensure the physical interface is up (no shutdown).
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 on R1 with the 'ip routing' global configuration command.
Why this is correct
The 'ip routing' command enables IP routing on a Cisco router, which is required for forwarding packets between subinterfaces in a router-on-a-stick configuration. Without it, the router will not route between VLANs.
Related concept
Access ports place end devices into a single VLAN.
✗
Configure the physical interface G0/0 with an IP address and enable trunking with 'switchport mode trunk'.
Why it's wrong here
In a router-on-a-stick configuration, the physical interface does not have an IP address; IP addresses are assigned to subinterfaces. Also, 'switchport mode trunk' is a switch command, not used on routers.
✗
Ensure the trunk between R1 and SW1 is configured with 'switchport trunk allowed vlan 10,20' on SW1.
Why it's wrong here
While this command is important on the switch side to allow VLANs on the trunk, the question focuses on R1's configuration. The issue described (missing 'ip routing') is on the router, not the switch.
✗
Add the 'no shutdown' command on R1's subinterfaces G0/0.10 and G0/0.20.
Why it's wrong here
Subinterfaces inherit the physical interface's state; they do not have their own 'shutdown' state. The 'no shutdown' command is only effective on the physical interface.
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 on R1 with the 'ip routing' global configuration command.Correct answer▾
Why this is correct
The 'ip routing' command enables IP routing on a Cisco router, which is required for forwarding packets between subinterfaces in a router-on-a-stick configuration. Without it, the router will not route between VLANs.
✗Configure the physical interface G0/0 with an IP address and enable trunking with 'switchport mode trunk'.Wrong answer — click to see why▾
Why this is wrong here
Assigning an IP to the physical interface would prevent subinterfaces from working, and 'switchport mode trunk' is invalid on a router interface.
Why candidates choose this
Candidates may think the physical interface needs an IP or trunk configuration, confusing router and switch commands.
✗Ensure the trunk between R1 and SW1 is configured with 'switchport trunk allowed vlan 10,20' on SW1.Wrong answer — click to see why▾
Why this is wrong here
The problem is specifically that R1 cannot route between VLANs due to missing 'ip routing', not a trunk pruning issue.
Why candidates choose this
Candidates often check trunk configuration first, but the symptom (no ping) could also be caused by trunk misconfiguration, leading them to overlook the router's routing capability.
✗Add the 'no shutdown' command on R1's subinterfaces G0/0.10 and G0/0.20.Wrong answer — click to see why▾
Why this is wrong here
Subinterfaces cannot be individually shut down; the physical interface controls the link state.
Why candidates choose this
Candidates may think subinterfaces need to be administratively enabled, similar to physical interfaces, but this is not the case.
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
In a router-on-a-stick configuration, the physical interface does not have an IP address; IP addresses are assigned to subinterfaces. Also, 'switchport mode trunk' is a switch command, not used on routers.
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 on R1 with the 'ip routing' global configuration command. — The problem is that R1 is missing the 'ip routing' command, which is required to enable IP routing on a router. Without it, R1 cannot forward packets between the subinterfaces. Additionally, the trunk between R1 and SW1 may not be operational because R1's physical interface G0/0 has no IP address and is not set to trunk mode (though subinterfaces handle encapsulation). The solution is to enable IP routing globally and ensure the physical interface is up (no shutdown).
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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