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  • Dynamic Trunking Protocol (Cisco)

    Design & Configure

    Dynamic Trunking Protocol (Cisco)

    Technology: Network

    Area: 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

    Author: Marcin Bialy
     
    Grandmetric