Dynamic Trunking Protocol (DTP)

DTP is a proprietary Cisco protocol for negotiating the encapsulation to use on the trunk link (ISL or 802.1q).

IEEE only support 802.1q as the trunk protocol and because of this there is no requirement to have IEEE version of DTP. Cisco have their own to trunking protocol (ie ISL) and this necessitates the need for DTP.

• DTP messages are sent every 30sec
• Default encapsulation selected is ISL then 802.1q
• To force a choice manually set the encapsulation

Mode	Sends Packet	Acts on Packet
Desirable	Yes	Yes
Auto	No	Yes	DEFAULT
On	No	Yes
Off	No	No	(no trunking)
No Neg	No	No	(trunking)

No Neg --> Sets the port to permanent trunking mode without sending or acting on DTP packets.


The table below shows what settings will allow a DTP to negotiate.

Either side of the link settings	Is truking negotiated?
Desirable / Desirable	Yes
Desirable / Auto	Yes
Desirable / On	Yes
Desirable / Off	No
Desirable / NoNeg	No
Auto / Auto	No
Auto / On	Yes
Auto / Off	No
Auto / NoNeg	No
On / On	Yes
On / Off	No
On / NoNeg	?
Off / Off	No
Off / NoNeg	No
NoNeg / NoNeg	?