Sunday, 19 February 2012

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
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 ?

1 comment:

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