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. The network has three departments: Sales (VLAN 10, 192.168.1.0/24), Engineering (VLAN 20, 192.168.2.0/24), and Management (VLAN 99, 192.168.99.0/24). A single switch SW1 connects to R1 via trunk interface G0/0. Subinterfaces for VLANs 10, 20, and 99 are already configured on R1 with correct encapsulation and IP addresses. However, inter-VLAN communication is failing. Troubleshoot and fix the configuration issue on R1 to enable routing between all VLANs.
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 192.168.1.1 255.255.255.0
!
interface GigabitEthernet0/0.20
encapsulation dot1Q 20
ip address 192.168.2.1 255.255.255.0
!
interface GigabitEthernet0/0.99
encapsulation dot1Q 99
ip address 192.168.99.1 255.255.255.0
!
R1#show running-config | include ip routing
no ip routing
R1#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/0 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/0 1-1005
Port Vlans allowed and active in management domain
Gi0/0 1,10,20,99
Port Native VLAN tagging enabled
Gi0/0 yes
A
Enable 'ip routing' globally on R1.
The router has correctly configured subinterfaces with 802.1Q encapsulation and IP addresses for VLANs 10, 20, and 99. However, 'ip routing' is disabled globally, so the router cannot forward packets between subinterfaces. Enabling 'ip routing' allows the router to route between VLANs.
B
Configure a subinterface for VLAN 1 and assign it an IP address.
Why wrong: This is incorrect because the native VLAN (VLAN 1) does not require a subinterface for inter-VLAN routing unless traffic needs to be routed to/from that VLAN. The main issue is that IP routing is disabled, not the lack of a VLAN 1 subinterface.
C
Change the encapsulation on the subinterfaces from dot1Q to ISL.
Why wrong: This is incorrect because 802.1Q is the standard trunking protocol used in modern networks. ISL is Cisco proprietary and outdated. The subinterfaces are correctly configured with dot1Q encapsulation.
D
Add a static route on R1 pointing to each VLAN subnet.
Why wrong: This is incorrect because the subinterfaces are directly connected networks, so routes are automatically added when IP addresses are configured. Static routes are not needed for directly connected networks. The real issue is that IP routing is disabled.
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 on R1.
The router has correctly configured subinterfaces with 802.1Q encapsulation and IP addresses for VLANs 10, 20, and 99. However, 'ip routing' is disabled globally, so the router cannot forward packets between subinterfaces. Additionally, the native VLAN on the trunk is VLAN 1, but no subinterface for VLAN 1 is configured, and the native VLAN mismatch could cause issues; however, the main problem is the lack of IP routing. Enable 'ip routing' globally and optionally configure a subinterface for the native VLAN if needed. The solution requires only 'ip routing' to enable inter-VLAN routing.
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 on R1.
Why this is correct
The router has correctly configured subinterfaces with 802.1Q encapsulation and IP addresses for VLANs 10, 20, and 99. However, 'ip routing' is disabled globally, so the router cannot forward packets between subinterfaces. Enabling 'ip routing' allows the router to route between VLANs.
Related concept
Access ports place end devices into a single VLAN.
✗
Configure a subinterface for VLAN 1 and assign it an IP address.
Why it's wrong here
This is incorrect because the native VLAN (VLAN 1) does not require a subinterface for inter-VLAN routing unless traffic needs to be routed to/from that VLAN. The main issue is that IP routing is disabled, not the lack of a VLAN 1 subinterface.
✗
Change the encapsulation on the subinterfaces from dot1Q to ISL.
Why it's wrong here
This is incorrect because 802.1Q is the standard trunking protocol used in modern networks. ISL is Cisco proprietary and outdated. The subinterfaces are correctly configured with dot1Q encapsulation.
✗
Add a static route on R1 pointing to each VLAN subnet.
Why it's wrong here
This is incorrect because the subinterfaces are directly connected networks, so routes are automatically added when IP addresses are configured. Static routes are not needed for directly connected networks. The real issue is that IP routing is disabled.
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 on R1.Correct answer▾
Why this is correct
The router has correctly configured subinterfaces with 802.1Q encapsulation and IP addresses for VLANs 10, 20, and 99. However, 'ip routing' is disabled globally, so the router cannot forward packets between subinterfaces. Enabling 'ip routing' allows the router to route between VLANs.
✗Configure a subinterface for VLAN 1 and assign it an IP address.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: The native VLAN does not need a subinterface; the router can still route traffic for other VLANs without it.
Why candidates choose this
Candidates pick this because they think the native VLAN mismatch or missing subinterface is the cause, but the trunk is correctly configured and the native VLAN mismatch is not the primary issue.
✗Change the encapsulation on the subinterfaces from dot1Q to ISL.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: ISL is not commonly used and would not fix the routing issue; the problem is IP routing disabled.
Why candidates choose this
Candidates pick this because they confuse trunking protocols or think encapsulation mismatch is the issue, but the switch and router both support dot1Q.
✗Add a static route on R1 pointing to each VLAN subnet.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: Directly connected routes are automatically added; no static routes are required.
Why candidates choose this
Candidates pick this because they think routing requires static routes, but the router already has the routes; it just cannot forward packets between 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.
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 on R1. — The router has correctly configured subinterfaces with 802.1Q encapsulation and IP addresses for VLANs 10, 20, and 99. However, 'ip routing' is disabled globally, so the router cannot forward packets between subinterfaces. Additionally, the native VLAN on the trunk is VLAN 1, but no subinterface for VLAN 1 is configured, and the native VLAN mismatch could cause issues; however, the main problem is the lack of IP routing. Enable 'ip routing' globally and optionally configure a subinterface for the native VLAN if needed. The solution requires only 'ip routing' to enable inter-VLAN routing.
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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