The answer is that VLAN 1 is not allowed on the trunk, so untagged frames are dropped. When a server sends untagged frames, the switch classifies them into the native VLAN of the trunk port, which by default is VLAN 1. However, if the trunk’s allowed VLAN list explicitly excludes VLAN 1—for instance, with the command `switchport trunk allowed vlan remove 1`—the switch will discard all frames belonging to that VLAN, including the untagged frames that would otherwise be placed into it. This scenario tests your understanding of native VLAN trunk configuration and how untagged frames interact with allowed VLAN lists, a common pitfall on the Cisco DCCOR 350-601 exam. A frequent trap is assuming untagged frames always pass regardless of the allowed list; in reality, the native VLAN must be permitted for them to be forwarded. Remember the mnemonic: “No native, no traffic—if VLAN 1 is banned, untagged frames are stranded.”
350-601 Network Practice Question
This 350-601 practice question tests your understanding of network. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
! Nexus 9000 switch
interface Ethernet1/1
switchport mode trunk
switchport trunk allowed vlan 10,20,30
spanning-tree port type edge trunk
no shutdown
! Switchport statistics
Ethernet1/1 is up, line protocol is up
Hardware: 1000/10000 Ethernet, address: xxxx.xxxx.xxxx
Description: Server connection
MTU 1500 bytes
Port mode is trunk
Trunk encapsulation is 802.1Q
Trunk allowed vlans: 10,20,30
Trunk native vlan: 1
Vlans allowed on trunk: 10,20,30
Vlans active and untagged: 1
Vlans active and tagged: 10,20,30
interface port-channel1
switchport mode trunk
switchport trunk allowed vlan 10,20,30
spanning-tree port type edge trunk
no shutdown
A server connected to Ethernet1/1 is unable to communicate on VLAN 1. The server is configured to send untagged frames. Based on the exhibit, what is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
! Nexus 9000 switch
interface Ethernet1/1
switchport mode trunk
switchport trunk allowed vlan 10,20,30
spanning-tree port type edge trunk
no shutdown
! Switchport statistics
Ethernet1/1 is up, line protocol is up
Hardware: 1000/10000 Ethernet, address: xxxx.xxxx.xxxx
Description: Server connection
MTU 1500 bytes
Port mode is trunk
Trunk encapsulation is 802.1Q
Trunk allowed vlans: 10,20,30
Trunk native vlan: 1
Vlans allowed on trunk: 10,20,30
Vlans active and untagged: 1
Vlans active and tagged: 10,20,30
interface port-channel1
switchport mode trunk
switchport trunk allowed vlan 10,20,30
spanning-tree port type edge trunk
no shutdown
A
The spanning-tree port type edge trunk is causing BPDU guard to block the port.
Why wrong: Port type edge trunk enables PortFast but does not block traffic on untagged VLAN.
B
The port is in err-disabled state due to a loop.
Why wrong: The port is up and line protocol is up.
C
VLAN 1 is not allowed on the trunk, so untagged frames are dropped.
Untagged frames are placed in native VLAN 1, but VLAN 1 is not in the allowed list.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 1 is not allowed on the trunk, so untagged frames are dropped.
The server sends untagged frames, which are placed into the native VLAN of the trunk port. By default, the native VLAN is VLAN 1. However, if VLAN 1 is explicitly removed from the allowed VLAN list on the trunk (e.g., with 'switchport trunk allowed vlan remove 1'), the switch will drop all frames belonging to that VLAN, including untagged frames that would otherwise be classified into VLAN 1. This matches the exhibit where the port is configured as a trunk but VLAN 1 is not allowed, causing the server's traffic to be dropped.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The spanning-tree port type edge trunk is causing BPDU guard to block the port.
Why it's wrong here
Port type edge trunk enables PortFast but does not block traffic on untagged VLAN.
✗
The port is in err-disabled state due to a loop.
Why it's wrong here
The port is up and line protocol is up.
✓
VLAN 1 is not allowed on the trunk, so untagged frames are dropped.
Why this is correct
Untagged frames are placed in native VLAN 1, but VLAN 1 is not in the allowed list.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The port is administratively down.
Why it's wrong here
The no shutdown command is present.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that untagged frames are always allowed on a trunk port, but the trap here is that the native VLAN must be explicitly permitted in the allowed VLAN list; otherwise, untagged frames are dropped even if the port is up and configured as a trunk.
Trap categories for this question
Command / output trap
The no shutdown command is present.
Detailed technical explanation
How to think about this question
On a Cisco switch trunk port, untagged frames are assigned to the native VLAN (default VLAN 1). If the native VLAN is removed from the allowed VLAN list using 'switchport trunk allowed vlan except 1' or 'remove 1', the switch will not forward any frames for that VLAN, including untagged traffic. This is a common misconfiguration when an administrator intends to restrict VLAN 1 for security reasons but forgets that the native VLAN must be allowed for untagged frames to pass. The 'show interfaces trunk' command would reveal that VLAN 1 is not listed in the allowed VLANs.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network — This question tests Network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: VLAN 1 is not allowed on the trunk, so untagged frames are dropped. — The server sends untagged frames, which are placed into the native VLAN of the trunk port. By default, the native VLAN is VLAN 1. However, if VLAN 1 is explicitly removed from the allowed VLAN list on the trunk (e.g., with 'switchport trunk allowed vlan remove 1'), the switch will drop all frames belonging to that VLAN, including untagged frames that would otherwise be classified into VLAN 1. This matches the exhibit where the port is configured as a trunk but VLAN 1 is not allowed, causing the server's traffic to be dropped.
What should I do if I get this 350-601 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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