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|
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||?|