This 200-301 practice question tests your understanding of connected to switch sw1. 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.
SW1# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 32768
Address 0011.2233.4455
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
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
------------------- ---- --- --------- -------- --------------------------------
Gi0/1 Desg FWD 4 128.1 P2p
Gi0/2 Desg FWD 4 128.2 P2p
SW1# show running-config | include bpduguard
spanning-tree portfast bpduguard default
SW1# show interfaces gigabitethernet 0/2
GigabitEthernet0/2 is down, line protocol is down (err-disabled)
Hardware is Gigabit Ethernet, address is 0011.2233.4456
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed
input flow-control is off, output flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped out
SW1# show errdisable recovery
ErrDisable Reason Timer Status
---------------- --------------
bpduguard Enabled
SW1# show spanning-tree vlan 20
VLAN0020
Spanning tree enabled protocol rstp
Root ID Priority 32778
Address 0055.6677.8899
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778
Address 0055.6677.8899
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1 Desg FWD 4 128.1 P2p
Gi0/2 Desg FWD 4 128.2 P2pYou are connected to switch SW1. Configure Rapid-PVST+ so that SW1 becomes the root bridge for VLAN 10 and VLAN 20. On interface GigabitEthernet0/2, enable PortFast and BPDUGuard. Then, a BPDU is received on that port, causing err-disable. Diagnose the issue and recover the interface without rebooting the switch.
Correct answer & explanation
First, configure SW1 as root for VLAN 10 and 20 using 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary'. Then, on interface Gi0/2, enable PortFast with 'spanning-tree portfast' and BPDUGuard with 'spanning-tree bpduguard enable'. The err-disable state occurs because BPDUGuard defaults to shutting down a port when a BPDU is received. To recover, use 'errdisable recovery cause bpduguard' to allow automatic recovery or 'shutdown' followed by 'no shutdown' on the interface. The blocking port is Gi0/2 in VLAN 20 because the current root has a higher priority than SW1's configured priority, but since SW1 becomes root, all ports are designated.
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.
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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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