questions about DTP

[simionov.adrian]

DTP is only the negociation of trunking, or also the negociation of access?

In Routing-Bits Handbook for CCIE I found that:

#switchport mode access

will not disable the DTP negociation. Only the nonegociate command will disable the DTP.

Is this true? This means that switchport mode access will still send DTP frames.

I can see on the switch that:

SW1#sh int ethernet 3/0 switchport | i Nego

Negotiation of Trunking: Off

when the port is mode access.

What about dynamic auto mode? Is sending DTP frames? Or is only accepting DTP frames in the negociation process?

[Peter Paluch · Accepted solution]

Hello Daniel,

DTP messages are sent in these circumstances:

• When the port is configured as dynamic desirable or dynamic auto
• When the port is configured as static trunk
• When the port first enters a static access mode (but not afterwards)

The first two options are self-explanatory, I believe - the DTP is sent if the port is in dynamic mode which is understandable, and it is also sent if the port works as a trunk.

The third option is somewhat curious. During my experiments, I have found out that if a port is configured as static access port, it will send a single DTP message, presumably to inform the other side of the link of the fact that it is entering the access mode. Afterwards, no more DTP messages are sent or accepted. Hence, an access port indeed does not send nor accept any DTP messages.

Best regards,

Peter

[Giuseppe Larosa]

Hello Daniel,

in dynamic auto mode the port will not start a DTP conversation, but it will answer to the other side of the link if the other side of the link starts to send DTP  L2 PDUs.

in access mode the port could answer stating it is in access mode so formally it still speaks DTP.

so in a "CCIE like" scenario if you are requested to have the port not speaking DTP you need also switchport nonegotiate

be aware that DTP is Cisco proprietary and it is not supported by Cisco routers.

also DTP introduces dependency from VTP in the sense that at VTP domain border DTP will not allow a trunk to be formed

Edit:

I would say Peter has provided a far better answer.

I last played with DTP many years ago ...

Hope to help

Giuseppe