Dymanic Trunking Protocol (DTP)

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 is Layer 2 protocol.
  • Multicast address of DTP is 01-00-0C-CC-CC.
  • Cisco proprietor so it only works on Cisco Switches.
  • DTP forwards 2 timer i.e Hello on every 30 sec and Hold is 300 sec.
  • DTP is default enable on all active interface of cisco switch.
  • DTP had 2 mode:
    1: Dymanic Auto (DA)
    2: Dymanic Desirable (DD).
  • BY default DTP negotiate two things:
    1: Trunking
    2: Encapsulation
  • 3750 platform switch performs Dynamic Auto (DA).
  • 3550 platform switch performs Dynamic Desirable (DD).

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 Modes
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:
Static and DTP Trunking