While making switch port trunk statically they do not negotiate with each other. To form Switchport trunk by negotation we use DTP.
Let's know few important point about DTP:
DTP forms switchport trunk by negotating to each other and basically negotiate trunking and encapsulation on port. As above point DTP has 2 modes: DD and DA.
Dymanic Auto (DA): DA is a mode of DTP which do not initiate negotiation with next port but if next port initate then it accapts and starts negotiation. Actually DA waits for initation to be perform by nest port to accept.
Dymanic Desirable (DD): DD is mode actually which start initating the negotation of trunking.
As we knew DA do not initate negotation of trunking but DD do. Let's see few scenerio for same:
DTP is Cisco proprietor and if default encapsulation is done and trunking performs then after trunking it will use ISL encapsulation but if you want to use Dot1Q then you have to change it manually.
Now, DTP has 2 Timer Hello and Hold. Now question comes what information are carried inside Hello Message?
DTP Hello message carries 3 information i.e DTP Mode, Trunking Encapsulation and VTP Domain Name which must match.
Let's see few more scenerios that arises during pratical implimentation: