The answer is that DTP is a Cisco proprietary protocol used to automatically negotiate trunk links between switches. This is correct because DTP operates at Layer 2 and relies on the native VLAN (VLAN 1) to exchange its frames; if the native VLANs are mismatched on either side, the negotiation can fail. On the ENCOR 350-401 exam, DTP properties are tested to ensure you understand which switchport modes actively initiate trunking versus passively respond—a common trap is confusing “dynamic desirable” (which actively sends DTP frames) with “dynamic auto” (which only responds). Remember that DTP is never used on routed ports or Layer 3 interfaces, and it only applies to switch ports in access or trunk mode. A useful memory tip: “Desirable sends, Auto awaits”—the dynamic desirable mode actively seeks a trunk, while dynamic auto waits for an invitation.
350-401 VLANs and Trunking Practice Question
This 350-401 practice question tests your understanding of vlans and trunking. 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.
Which three statements about Dynamic Trunking Protocol (DTP) are true? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
DTP is a Cisco proprietary protocol.
Correct: A is true because DTP is Cisco proprietary and not supported on non-Cisco switches. B is true because DTP uses VLAN 1 (the native VLAN) for its frames; if the native VLAN is mismatched, DTP may fail. C is true because the 'switchport mode dynamic desirable' interface configuration command actively sends DTP frames to negotiate trunking. D is incorrect because DTP operates at Layer 2, not Layer 3. E is incorrect because DTP is not used on routed ports; it only applies to switch ports in access or trunk mode.
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.
✓
DTP is a Cisco proprietary protocol.
Why this is correct
Correct because DTP is only available on Cisco switches and is not standardized.
Related concept
Access ports place end devices into a single VLAN.
✓
DTP frames are sent on the native VLAN.
Why this is correct
Correct because DTP uses the native VLAN (typically VLAN 1) to exchange negotiation messages.
Related concept
Access ports place end devices into a single VLAN.
✓
The 'switchport mode dynamic desirable' setting causes the interface to actively attempt to form a trunk.
Why this is correct
Correct because dynamic desirable mode sends DTP frames to encourage the neighbor to become a trunk.
Related concept
Access ports place end devices into a single VLAN.
✗
DTP operates at Layer 3 of the OSI model.
Why it's wrong here
Incorrect because DTP is a Layer 2 protocol used for trunk negotiation.
✗
DTP is used to negotiate trunking on routed ports.
Why it's wrong here
Incorrect because DTP only applies to Layer 2 switch ports; routed ports do not participate in DTP.
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 350-401 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 350-401 questions on switching, trunking, and access-port configuration.
VLANs and Trunking — This question tests VLANs and Trunking — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: DTP is a Cisco proprietary protocol. — Correct: A is true because DTP is Cisco proprietary and not supported on non-Cisco switches. B is true because DTP uses VLAN 1 (the native VLAN) for its frames; if the native VLAN is mismatched, DTP may fail. C is true because the 'switchport mode dynamic desirable' interface configuration command actively sends DTP frames to negotiate trunking. D is incorrect because DTP operates at Layer 2, not Layer 3. E is incorrect because DTP is not used on routed ports; it only applies to switch ports in access or trunk mode.
What should I do if I get this 350-401 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 350-401 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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which two statements about DTP (Dynamic Trunking Protocol) are true? (Choose two.)
medium
✓ A.DTP is a Cisco proprietary protocol.
✓ B.The default switchport mode on a Cisco Catalyst switch is dynamic desirable.
C.DTP frames are sent continuously on a trunk port to maintain the trunk.
D.The 'switchport nonegotiate' command enables DTP on an interface.
E.DTP supports both 802.1Q and ISL trunking encapsulation.
Why A: DTP is a Cisco proprietary protocol used to negotiate trunking between switches. The default mode on Cisco switches is dynamic desirable, which will actively try to form a trunk. DTP frames are sent only over access ports when trunking is being negotiated, but not over trunk ports once the trunk is established. The 'switchport nonegotiate' command disables DTP, and trunk formation then relies on manual configuration.
Variation 2. Drag and drop each DTP mode on the left to its matching trunking behavior on the right.
medium
✓ P1.dynamic desirable: Actively sends DTP frames to form trunk
✓ P2.dynamic auto: Passively waits for DTP frames to form trunk
✓ P3.trunk: Forces the port to become a trunk regardless of neighbor
✓ P4.access: Forces the port to be an access port, no trunking
✓ P5.non-negotiate: Disables DTP, trunking only if manually configured
Why P1: Dynamic desirable actively negotiates trunking; dynamic auto waits for a neighbor to initiate; trunk forces trunking; access forces access mode; non-negotiate disables DTP.
Variation 3. Drag and drop each DTP mode on the left to its matching trunking behavior on the right.
medium
✓ P1.dynamic desirable: Actively sends DTP frames; forms trunk if neighbor is trunk, desirable, or auto
✓ P2.dynamic auto: Passively waits for DTP frames; forms trunk only if neighbor is trunk or desirable
✓ P3.trunk: Forces the interface to be a trunk regardless of DTP negotiation
✓ P4.access: Forces the interface to be an access port; never forms a trunk
Why P1: Dynamic desirable actively sends DTP frames and forms a trunk if the neighbor is trunk, dynamic desirable, or dynamic auto. Dynamic auto forms a trunk only if the neighbor is trunk or dynamic desirable. Trunk always forms a trunk. Access never forms a trunk. None disables DTP.
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