This 200-301 practice question tests your understanding of 200-301 exam topics. 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.
R1#show running-config | section interface
interface GigabitEthernet1/0/1
description Uplink to Core
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet1/0/2
description Uplink to Core
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet1/0/3
description Potential Root Port
switchport access vlan 10
switchport mode access
spanning-tree guard root
!
interface GigabitEthernet1/0/4
description Server
switchport access vlan 10
switchport mode access
spanning-tree portfast
!
interface GigabitEthernet1/0/5
description Desktop
switchport access vlan 10
switchport mode access
spanning-tree portfast
spanning-tree bpduguard enable
!
interface Vlan10
ip address 192.168.10.1 255.255.255.0
!
R1#show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 4097
Address 0011.2233.4455
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 4097 (priority 4096 sys-id-ext 1)
Address 0011.2233.4455
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 4 128.1 Shr
Gi1/0/2 Desg FWD 4 128.2 Shr
Gi1/0/3 Desg BKN*4 128.3 Shr *ROOT_Guard
Gi1/0/4 Desg FWD 4 128.4 Shr Edge
Gi1/0/5 Desg FWD 4 128.5 Shr Edge
R1#show interfaces status | include err-disabled
Gi1/0/5 err-disabled auto auto 10/100/1000You are connected to R1, a multilayer switch operating as a root bridge for VLAN 10. Configure Root Guard on port GigabitEthernet1/0/3 to protect the root bridge role, enable Loop Guard on the uplink interfaces GigabitEthernet1/0/1 and GigabitEthernet1/0/2, and apply BPDU Guard on all PortFast-enabled interfaces. After configuration, a superior BPDU is received on G1/0/3, placing it in a blocked state, and a host connected to G1/0/5 triggers BPDU Guard, causing err-disable. Troubleshoot and resolve by adjusting configurations.
Correct answer & explanation
The candidate must first identify that Root Guard is correctly configured on G1/0/3, causing it to block (BKN*ROOT_Guard) upon receiving a superior BPDU, which is correct behavior. However, the task states to protect the root bridge role; since R1 is already root, Root Guard is appropriate. The err-disabled port G1/0/5 indicates BPDU Guard triggered; this is expected because a host connected to a PortFast port sent a BPDU. To resolve, the candidate should re-enable the port with 'no shutdown' and ensure BPDU Guard is properly applied. Additionally, Loop Guard is missing on uplinks G1/0/1 and G1/0/2; it must be configured with 'spanning-tree guard loop' under each interface. No changes to Root Guard are needed; the blockage is intentional.
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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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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