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DTP (Dynamic Trunking Protocol) — Key Facts and Modes
Which three statements about Dynamic Trunking Protocol (DTP) are true? (Choose three.)
Quick Answer
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.
⚠ Common exam trap
A common mix-up: candidates confuse DTP with protocols that operate at Layer 3 or assuming it works on routed ports; candidates might think DTP negotiates trunking on any port type, but it is strictly for switch ports (Layer 2).
Answer choices
Why each option matters
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.
Option A is correct because DTP (Dynamic Trunking Protocol) is a Cisco proprietary protocol used to negotiate trunk encapsulation and trunking mode between directly connected Cisco switches. Option B is correct because DTP frames are sent untagged on the native VLAN (VLAN 1 by default), which is why both ends must agree on the native VLAN for DTP to function properly. Option C is correct because 'switchport mode dynamic desirable' makes the interface actively initiate DTP negotiation and form a trunk if the neighbor responds with trunk, desirable, or auto mode. Option D is incorrect because DTP operates at Layer 2 (data link layer), not Layer 3. Option E is incorrect because DTP negotiates trunking on switch ports, not routed ports; routed ports are configured with 'no switchport' and do not run DTP.
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
DTP is Cisco proprietary, so it only negotiates trunking between Cisco switches; non-Cisco devices ignore its frames. This satisfies the stem's requirement for a true statement, since standard 802.1Q lacks any negotiation mechanism, and DTP's proprietary nature is a defining characteristic tested on the 350-401 exam.
- ✓
DTP frames are sent on the native VLAN.
Why this is correct
DTP frames are transmitted untagged on the native VLAN, since trunk negotiation must occur before 802.1Q tagging is established between switches. This satisfies the scenario's requirement that DTP operate on the access VLAN of the link, allowing neighbouring Cisco switches to negotiate trunking mode before any tagged traffic flows.
- ✓
The 'switchport mode dynamic desirable' setting causes the interface to actively attempt to form a trunk.
Why this is correct
Dynamic desirable places the interface in an active negotiation state, transmitting DTP frames to request trunking and forming one if the neighbour responds with desirable, auto or trunk mode. This satisfies the stem's description of actively attempting to form a trunk.
- ✗
DTP operates at Layer 3 of the OSI model.
Why it's wrong here
DTP is a Layer 2 protocol that negotiates trunk encapsulation and mode on switch ports; it carries no Layer 3 addressing or routing function. Confusing it with routed protocols is tempting because trunk links often carry multiple VLANs, but DTP frames themselves never operate above the data link layer.
- ✗
DTP is used to negotiate trunking on routed ports.
Why it's wrong here
DTP negotiates trunking only on switch access ports; routed ports have no trunking state to negotiate and are configured manually. The confusion is tempting because trunk links commonly interconnect Layer 3 devices, but DTP's negotiation applies to Layer 2 switchport interfaces, not routed ones.
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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Same concept, more angles
3 more ways this is tested on 350-401
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: Option A is correct because DTP (Dynamic Trunking Protocol) is a Cisco proprietary Layer 2 protocol used to negotiate trunking between Cisco switches, and it is not an IEEE standard. Option B is correct because the factory default switchport mode on Cisco Catalyst switches is dynamic desirable, meaning the port actively attempts to convert the link to a trunk by sending DTP frames. Option C is incorrect because DTP frames are not sent continuously to maintain a trunk; they are exchanged during negotiation, and once a trunk is established, DTP messages are sent periodically (every 30 seconds) only if the port is in a dynamic mode, not to keep the trunk alive. Option D is incorrect because the 'switchport nonegotiate' command disables DTP on an interface, preventing the port from sending DTP frames, rather than enabling it. Option E is incorrect because DTP itself does not support or negotiate encapsulation types; it only negotiates whether a link becomes a trunk, while the encapsulation (802.1Q or ISL) must be configured separately.
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
- ✓ P5.none (nonnegotiate): Disables DTP; requires manual trunk configuration
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.
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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