Dynamic Trunking Protocol (Cisco)

Technology: Switching

Vendor: Cisco

Title: DTP – Dynamic Trunking Protocol

Software: 12.X , 15.X

Platform: Catalyst 2960, 3560, 3750, 3850, 4500, 6500

Cisco proprietary protocol used to automatically negotiate the formation of Trunk between two switches. Port can be configured in one of the below states:

• Dynamic auto – passive mode, allow to form “Trunk” state if second end is actively negotiating;
• Dynamic desirable – active mode, allow to form “Trunk”;
• Trunk – put interface in permanent “Trunk” mode;
• Access – DTP protocol is desirably in off state.

Additionally in permanent “Trunk” mode we could switch off the Dynamic Trunking Protocol by using “switchport nonegotiate” command.

To configure the port in passive DTP mode waiting for active peer to form the “Trunk”:

SW1#configure terminal
SW1(config)#interface gigabitEthernet 1/0/20
SW1(config-if)#switchport mode dynamic auto

To configure the port in active DTP mode to willingly form the “Trunk”:

SW1#configure terminal
SW1(config)#interface gigabitEthernet 1/0/20
SW1(config-if)#switchport mode dynamic desirable

To configure the port in “Trunk” mode and doesn’t engage in negotiation over DTP:

SW1#configure terminal
SW1(config)#interface gigabitEthernet 1/0/20
SW1(config-if)#switchport mode trunk
SW1(config-if)#switchport nonegotiate

To configure the port in “Access” mode and doesn’t use DTP:

SW1#configure terminal
SW1(config)#interface gigabitEthernet 1/0/20
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 100

Verification commands:

Link status:

TOS – Trunk Operational Status
TAS – Trunk Administrative Status
TNS – Trunk Negotiation Status
TOT – Trunk Operational Encapsulation Type
TAT – Trunk Administrative Encapsulation Type
TNT – Trunk Negotiation Encapsulation Type

SW1#show dtp interfaces gi1/0/20

c) Port in permanent “Trunk” state, sending DTP packets over the link.

DTP information for GigabitEthernet1/0/20:

TOS/TAS/TNS:                              TRUNK/ON/TRUNK
TOT/TAT/TNT:                              802.1Q/802.1Q/802.1Q

d) Port in permanent “Trunk” state, not sending DTP packets over the link.

DTP information for GigabitEthernet1/0/20:

TOS/TAS/TNS:                              TUNK/NONEGOTIATE/TRUNK
TOT/TAT/TNT:                              802.1Q/802.1Q/802.1Q

e) Port in permanent “Access” state, not sending DTP packets over the link.

DTP information for GigabitEthernet1/0/20:

TOS/TAS/TNS:                              ACCESS/OFF/ACCESS
TOT/TAT/TNT:                              NATIVE/802.1Q/NATIVE

a) Port in dynamic desirable state, sending DTP packets over the link

DTP information for GigabitEthernet1/0/20:

TOS/TAS/TNS:                              TRUNK/DESIRABLE/TRUNK
TOT/TAT/TNT:                              802.1Q/802.1Q/802.1Q

b) Port in dynamic auto state, sending DTP packets over the link

DTP information for GigabitEthernet1/0/20:

TOS/TAS/TNS:                              TRUNK/AUTO/TRUNK
TOT/TAT/TNT:                              802.1Q/802.1Q/802.1Q