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 via console. R1 is a router-on-a-stick connecting VLAN 10 and VLAN 20 on a single link to switch SW1. Currently, hosts in VLAN 10 cannot ping hosts in VLAN 20, and some VLAN 10 hosts report intermittent connectivity. Examine the provided configuration and output, then fix all issues to restore full inter-VLAN routing and stable trunk operation.
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 203.0.113.2 255.255.255.252
duplex auto
speed auto
!
router ospf 1
network 203.0.113.0 0.0.0.3 area 0
!
SW1# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gig0/1 on 802.1q trunking 99
Port Vlans allowed on trunk
Gig0/1 1-1005
Port Vlans allowed and active in management domain
Gig0/1 10,20
SW1# show running-config | include vlan
vlan 10
vlan 20
vlan 99
A
On R1, configure 'ip routing', add subinterface G0/0.99 with 'encapsulation dot1Q 99 native' and an IP address, and ensure G0/0 is not shutdown.
This corrects all three issues: enables inter-VLAN routing globally, creates a native VLAN subinterface matching SW1's native VLAN 99, and ensures the physical interface is active.
B
On R1, configure 'ip routing' and change the encapsulation on the VLAN 10 subinterface to 'encapsulation dot1Q 10 native'.
Why wrong: This is incorrect because making VLAN 10 the native VLAN does not match SW1's native VLAN 99, causing a native VLAN mismatch that drops untagged frames.
C
On R1, configure 'ip routing' and add an IP address to the physical interface GigabitEthernet0/0.
Why wrong: This is incorrect because assigning an IP to the physical interface is not needed for router-on-a-stick; subinterfaces handle VLAN traffic. Also, the native VLAN issue remains.
D
On R1, configure 'ip routing' and change the encapsulation on the VLAN 20 subinterface to 'encapsulation dot1Q 20 native'.
Why wrong: This is incorrect because making VLAN 20 native does not match SW1's native VLAN 99, and the native VLAN mismatch persists.
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 'ip routing', add subinterface G0/0.99 with 'encapsulation dot1Q 99 native' and an IP address, and ensure G0/0 is not shutdown.
Three issues prevent inter-VLAN routing and cause intermittent connectivity. First, R1 lacks a subinterface configured for the native VLAN (VLAN 99) with the 'encapsulation dot1Q 99 native' command; this subinterface is needed because SW1 expects the native VLAN to be tagged on the router side for consistency. Second, the global command 'ip routing' is missing, which disables routing between VLANs. Third, the physical interface GigabitEthernet0/0 might be administratively down; ensure 'no shutdown' is applied. Option A correctly fixes all issues: it adds the native VLAN subinterface, enables routing, and ensures the interface is up. The other options fail because they either misconfigure the native VLAN encapsulation on the wrong subinterface or apply an IP address to the physical interface (which does not support routing on a router-on-a-stick).
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 'ip routing', add subinterface G0/0.99 with 'encapsulation dot1Q 99 native' and an IP address, and ensure G0/0 is not shutdown.
Why this is correct
This corrects all three issues: enables inter-VLAN routing globally, creates a native VLAN subinterface matching SW1's native VLAN 99, and ensures the physical interface is active.
Related concept
Access ports place end devices into a single VLAN.
✗
On R1, configure 'ip routing' and change the encapsulation on the VLAN 10 subinterface to 'encapsulation dot1Q 10 native'.
Why it's wrong here
This is incorrect because making VLAN 10 the native VLAN does not match SW1's native VLAN 99, causing a native VLAN mismatch that drops untagged frames.
✗
On R1, configure 'ip routing' and add an IP address to the physical interface GigabitEthernet0/0.
Why it's wrong here
This is incorrect because assigning an IP to the physical interface is not needed for router-on-a-stick; subinterfaces handle VLAN traffic. Also, the native VLAN issue remains.
✗
On R1, configure 'ip routing' and change the encapsulation on the VLAN 20 subinterface to 'encapsulation dot1Q 20 native'.
Why it's wrong here
This is incorrect because making VLAN 20 native does not match SW1's native VLAN 99, and the native VLAN mismatch persists.
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 'ip routing', add subinterface G0/0.99 with 'encapsulation dot1Q 99 native' and an IP address, and ensure G0/0 is not shutdown.Correct answer▾
Why this is correct
This corrects all three issues: enables inter-VLAN routing globally, creates a native VLAN subinterface matching SW1's native VLAN 99, and ensures the physical interface is active.
✗On R1, configure 'ip routing' and change the encapsulation on the VLAN 10 subinterface to 'encapsulation dot1Q 10 native'.Wrong answer — click to see why▾
Why this is wrong here
The native VLAN must match on both ends; SW1 uses VLAN 99 as native, so R1 must use VLAN 99 as native, not VLAN 10.
Why candidates choose this
Candidates may think the native VLAN should match the VLAN being routed, but it must match the switch's native VLAN.
✗On R1, configure 'ip routing' and add an IP address to the physical interface GigabitEthernet0/0.Wrong answer — click to see why▾
Why this is wrong here
Router-on-a-stick uses subinterfaces, not the physical interface IP. The physical interface only needs to be up and trunking.
Why candidates choose this
Candidates might think the physical interface needs an IP to route, but in dot1q trunking, subinterfaces carry the IPs.
✗On R1, configure 'ip routing' and change the encapsulation on the VLAN 20 subinterface to 'encapsulation dot1Q 20 native'.Wrong answer — click to see why▾
Why this is wrong here
The native VLAN must be consistent across the trunk; SW1 uses VLAN 99, so R1 must use VLAN 99 as native.
Why candidates choose this
Candidates might assume any VLAN can be native, but the switch's native VLAN must match; otherwise, frames are misclassified.
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 'ip routing', add subinterface G0/0.99 with 'encapsulation dot1Q 99 native' and an IP address, and ensure G0/0 is not shutdown. — Three issues prevent inter-VLAN routing and cause intermittent connectivity. First, R1 lacks a subinterface configured for the native VLAN (VLAN 99) with the 'encapsulation dot1Q 99 native' command; this subinterface is needed because SW1 expects the native VLAN to be tagged on the router side for consistency. Second, the global command 'ip routing' is missing, which disables routing between VLANs. Third, the physical interface GigabitEthernet0/0 might be administratively down; ensure 'no shutdown' is applied. Option A correctly fixes all issues: it adds the native VLAN subinterface, enables routing, and ensures the interface is up. The other options fail because they either misconfigure the native VLAN encapsulation on the wrong subinterface or apply an IP address to the physical interface (which does not support routing on a router-on-a-stick).
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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