Dynamic Trunking Protocol ( DTP )

DTP is used to negotiate forming a trunk between two cisco devices. DTP is a Cisco proprietor protocol that is automatically enabled on Catalyst 2960 and Catalyst 3560 Series switches 

The default switchport mode for newer Cisco switch Ethernet interfaces is dynamic auto

Note that if two Cisco switches are left to the common default setting of auto, a trunk will never form. switchport mode dynamic desirable: Makes the interface actively attempt to convert the link to a trunk link. please refer  the DTP modes below carefully 

DTP modes:

1. switchport mode access -  Just to add a computer on the switch 

2. switchport mode dynamic auto - Makes the interface able to convert the link into a trunk link, when someone ask/request this port, then trunking will be negotiated. 

3. switchport mode dynamic desirable -  Makes the interface actively attempt to convert the link into a trunk link. it " asks" the far end switch to bring up a trunk

4. switchport mode trunk - Puts the interface into permanent trunking modes and negotiates to convert the neighboring link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface. For this mode there will be no such condition, rather it accepts whatever the mode neighbor is.

5. switchport nonegotiate -  Prevents the interface from generating DTP frames. you can use this command only when the interface switchport mode is access or trunk.you must manually configure the neighboring interface as a trunk interface to establish a trunk link.

                                 Please refer the below table for quick understanding

                           sh interface gigbitether 2/1 trunkc to check the trunking status                                  

Key Points -

DTP frames are sent out every 30 seconds to ensure their neighboring switch port informed of the links’s mode

·     Switch port truck will will be the always first and best choice making both switch connected with swichport mode truck manually, so that both of them will never ask for negotiation.

When should I make truck between two switches - A trunk link can be negotiated between two switches only at following conditions.

  If both switches belonging to the same VTP domain

  If one switch has not been defined VTP domain, between two of them

  If both switches have not been defined VTP domain

  If two switches are in different VTP domains, though trunking can be possible, by typing 

  switchport mode trunk and switchport nonegotiate on both of the switches.

When should I disable DTP -

  

Whenever you made permanent trunk link with switchport mode trunk, you should also disable DTP completely so that these frame are not changed, hence no negotiation is possible until this configured removed.

You should disable DTP negotiation if a switch has a trunk link connected to a non-trunking router or firewall interface.



Note - Please refer another page for VTP explanation