Quiz Entry - updated: 2026.07.14
What is DTP (Dynamic Trunking Protocol), and what are the four switchport modes?
DTP is a Cisco proprietary protocol that automatically negotiates whether a link becomes a trunk or stays as access. The four modes are: access, dynamic auto, dynamic desirable, and trunk.
| Mode | Behavior |
|---|---|
| access | Permanent access mode. Actively sends DTP frames to tell the neighbor to be access too |
| dynamic auto | Passively waits — only becomes a trunk if the other side is set to trunk or dynamic desirable |
| dynamic desirable | Actively tries to form a trunk — succeeds if the other side is trunk, desirable, or auto |
| trunk | Permanent trunk mode. Sends DTP frames to convince the neighbor to trunk as well |
DTP characteristics:
- Enabled by default on Catalyst 2960/2950 switches
- Default mode is dynamic auto (passive — won't form a trunk unless the neighbor initiates)
- Use
switchport nonegotiateto disable DTP on a port (stops sending DTP frames) - Re-enable DTP with
switchport mode dynamic auto
Security best practice: Always explicitly set ports to either access or trunk and use switchport nonegotiate on trunk ports. DTP negotiation is an attack vector — an attacker could trick a dynamic port into becoming a trunk and gain access to all VLANs (Virtual Local Area Networks).
Go deeper:
Dynamic Trunking Protocol (Wikipedia) — defines DTP and distinguishes the administrative modes (access, trunk, dynamic auto, dynamic desirable) from the negotiated operational mode.