DTP Packet headers

[jpduminy]

Hi All,

I wanted to understand why switch wont form a Trunk if both sides are dynamic auto.
I have done the simulation lab and can see even dynamic auto sends a negotiation message.

Just wanted to understand what headers switch will check to make the port trunk.

Note :- i knew for trunk it is mandatory to configure mode on or desirable.

I just wanted to understand what information available in desirable frames which is not available in auto frames.

[M02@rt37]

Hello @jpduminy 

Dynamic auto sends a negotiation message but only "hello" or advertisement-type DTP messages, not negotiation requests...

Mode is passive, it’s not enough on its own, and two autos will just wait forever.

DTP uses EtherType 0x2004 and is cisco proprietary. The switch check in the DTP header the domain name (must match on both sides), the DTp version, and Trunk status (access/trunk/negotiating).