About Lesson
Dynamic Trunking Protocol (DTP)
- Dynamic trunk ports are established by the switch port sending DTP packets to negotiate whether the other end can be a trunk port.
- Advertises itself every 30 seconds to neighbors so that they are kept aware of its status.
- Requires that the VTP domain match between the two switches.
Dynamic Trunking Protocol (DTP) Modes
- Trunk – switchport mode trunk statically places the switch port as a trunk. This mode advertises DTP packets to the other end to establish a dynamic trunk.
- Dynamic desirable – Using switchport mode dynamic desirable, the switch port acts as an access port, but listens for and advertises DTP packets to the other end to establish a dynamic trunk. If it is successful in negotiation, the port becomes a trunk port.
- Dynamic auto: Using the command switchport mode dynamic auto, the switch port acts as an access port, but it listens for DTP packets. It responds to DTP packets and upon successful negotiation the port becomes a trunk port.
| Terms | Trunk | Dynamic Desirable | Dynamic Auto |
| Trunk | ✓ | ✓ | ✓ |
| Dynamic Desirable | ✓ | ✓ | ✓ |
| Dynamic Auto | ✓ | ✓ | X |
Dynamic Trunking Protocol (DTP) Mode Configuration
The trunk port status is verified with the command show interface [interface-id] trunk.
DTP Mode Off Configuration
A static trunk port attempts to establish and negotiate a trunk port with a neighbor by default. However, the interface configuration command switchport nonegotiate prevents that port from forming a trunk port. Negotiation of Trunk now displays as Off. 
Note: As a best practice, configure both ends of a link as a fixed port type (using switchport mode access or switchport mode trunk) to remove any uncertainty about the port’s operations.
Other useful information:
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