Dynamic Trunking Protocol or DTP is a dynamic Layer 2 trunking protocol developed by Cisco. If a port can become a trunk Dynamic Trunking Protocol may also have the ability to trunk automatically, and in some cases even negotiate what type of trunking to use on the port. DTP provides this ability to negotiate the trunking method with the other device.
Dynamic Mode Combinations
dynamic desirable < – – – – – – – – – > nonegotiate
dynamic desirable < – – – – – – – – – > dynamic auto
dynamic auto < – – – – X – – – – > dynamic auto
nonegotiate < – – – – X – – – – > dynamic auto
access < – – – – X – – – – > dynamic desirable
access < – – – – X – – – – > dynamic auto
Operational Status
- TOS = Trunk Operational Status
- TAS = Trunk Administrative Status
- TNS = Trunk Negotiation Status
- TOT = Trunk Operational Type
- TAT = Trunk Administrative Type
- TNT = Trunk Negotiation Type
Always Trunk and send DTP messages to other Switch
Switch#configure terminal
Switch(config)#interface GigabitEthernetx/y/z
Switch(config-if)#switchport mode trunk
Switch(config-if)#end
Switch#
Always Trunk does NOT send DTP messages to other Switch
Switch#configure terminal
Switch(config)#interface GigabitEthernetx/y/z
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport nonegotiate
Switch(config-if)#end
Switch#
Send DTP messages and forms a Trunk if negotiation is successful
Switch#configure terminal
Switch(config)#interface GigabitEthernetx/y/z
Switch(config-if)#switchport mode dynamic desirable
Switch(config-if)#end
Switch#
Replies to DTP messages and forms a Trunk if negotiation is successful
Switch#configure termianl
Switch(config)#interface GigabitEthernetx/y/z
Switch(config-if)#switchport mode dynamic auto
Switch(config-if)#end
Switch#
Never Trunks and sends DTP messages to help other Switch
Switch#configure terminal
Switch(config)#interface GigabitEthernetx/y/z
Switch(config-if)#switchport mode access
Switch(config-if)#end
Switch#
Never Trunks and does not send DTP messages to other Switch
Switch#configure terminal
Switch(config)#interface GigabitEthernetx/y/z
Switch(config-if)#switchport mode access
Switch(config-if)#switchport nonegotiate
Switch(config-if)#end
Switch#
My personal rule of thumb regarding the use of DTP is to turn it off on Trunks ports. I prefer to have total control of the Trunk ports between Switches, as well as the Trunk ports between Switches and Routers. I also disable DTP between Switches and Wireless LAN Controllers. Basically any port that needs to encapsulate Layer 2 VLAN information in the ethernet header.
