This 200-301 practice question tests your understanding of connected to router r3 via the console. 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.
R3# show running-config | section interface GigabitEthernet0/0/0
interface GigabitEthernet0/0/0
no ip address
duplex auto
speed auto
!
R3# show running-config | section interface GigabitEthernet0/0/0.10
interface GigabitEthernet0/0/0.10
encapsulation dot1Q 10
ip address 10.10.10.1 255.255.255.0
!
R3# show running-config | section interface GigabitEthernet0/0/0.20
interface GigabitEthernet0/0/0.20
encapsulation dot1Q 20
ip address 10.10.20.1 255.255.255.0
!
R3# 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
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, GigabitEthernet0/0/0.10
L 10.10.10.1/32 is directly connected, GigabitEthernet0/0/0.10
C 10.10.20.0/24 is directly connected, GigabitEthernet0/0/0.20
L 10.10.20.1/32 is directly connected, GigabitEthernet0/0/0.20
R3# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gig0/0/0 auto n-802.1q trunking 1
Port Vlans allowed on trunk
Gig0/0/0 1-4094
Port Vlans allowed and active in management domain
Gig0/0/0 1
Port Vlans in spanning tree forwarding state and not pruned
Gig0/0/0 1You are connected to Router R3 via the console. R3 is a Cisco 4321 running IOS-XE. There is a switch S1 connected to R3 G0/0/0, and two VLANs (10 and 20) exist on S1. S1's G0/1 is an access port in VLAN 10 for PC-A (10.10.10.10/24), and G0/2 is an access port in VLAN 20 for PC-B (10.10.20.20/24). However, PC-A and PC-B cannot ping each other. The network engineer suspects a misconfiguration on R3. Inspect the provided configuration and output, then correct the issue(s) so that inter-VLAN routing works between the two PCs.
Correct answer & explanation
The issue is a native VLAN mismatch. The switch S1 likely has its trunk port configured with native VLAN 10 (common for such setups), but R3's physical interface GigabitEthernet0/0/0 still uses the default native VLAN 1. This causes the subinterface for VLAN 10 to not pass untagged traffic correctly, and the trunk shows only VLAN 1 as allowed and active on R3. The fix is to configure the native VLAN on R3's trunk to match the switch's expectation. First, on the physical interface, set the native VLAN to 10 using 'encapsulation dot1Q 10 native'. Then, re-add the native VLAN subinterface (or ensure it is present) with appropriate IP. Additionally, verify that 'ip routing' is enabled (it is, since routes appear). The solution commands adjust the trunk encapsulation and subinterface.
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.
Common exam traps
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
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.
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
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.
What to study next
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.
Related practice questions
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
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Practise VLANs, access ports, trunks, allowed VLANs and switching scenarios.
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Practise standard and extended ACLs, permit/deny logic and traffic filtering.
Practise static NAT, dynamic NAT, PAT and inside/outside address translation.
Practise DHCP scopes, relay, leases and troubleshooting.
Practise routing-table output, longest-prefix match, AD and route selection.
Practise trunk verification and VLAN forwarding across switches.
Practise WLAN security, authentication and wireless architecture concepts.
Practise IPv6 addressing, routes, neighbour discovery and common IPv6 exam traps.
Practice this exam
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Access ports place end devices into a single VLAN.
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.
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.
Access ports place end devices into a single VLAN.
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