Dynamic Trunking Protocol is a Cisco proprietary protocol used to negotiate trunks between Cisco devices. To autonegotiate trunking, the interfaces must be in the same VLAN Trunking Protocol (VTP) domain. Trunk negotiation is managed by the Dynamic Trunking Protocol, which is a point-to-point protocol.
Table of Contents
Configure
Switch# configure terminal
Switch(config)# interface interface-id
Switch(config-if)# switchport mode {access | dynamic {auto | desirable} | trunk}
Modes
- access: Set the port to access mode. The port is set to access unconditionally and operates as a nontrunking, single VLAN interface that sends and receives nonencapsulated (non-tagged) frames. An access port can be assigned to only one VLAN.
- dynamic auto: Set the interface trunking mode dynamic parameter to auto to specify that the interface convert the link to a trunk link. This is the default switchport mode.
- dynamic desirable: Set the interface trunking mode dynamic parameter to desirable to specify that the interface actively attempt to convert the link to a trunk link.
- trunk: Set the port to trunk unconditionally. The port is a trunking VLAN Layer 2 interface. The port sends and receives encapsulated (tagged) frames that identify the VLAN of origination. A trunk is a point-to-point link between two switches or between a switch and a router.
Verify
Switch# show interfaces interface-id switchport Name: interface-id Switchport: Enabled Administrative Mode: dynamic auto Operational Mode: static access Administrative Trunking Encapsulation: negotiate Operational Trunking Encapsulation: native Negotiation of Trunking: On
Troubleshoot
The default switchport mode is dynamic auto, so if two switches are connected, they will not form a trunk.
Access mode will not trunk, period.
Trunk mode will change the interface to permanent trunking mode.
Dynamic auto will only trunk if the neighboring interface is set to trunk or desirable mode.
Dynamic desirable will trunk if the neighboring interface is set to trunk, desirable, or auto mode.
References
- https://networklessons.com/switching/cisco-dtp-dynamic-trunking-protocol-negotiation/
- https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/15-0_1_se/command/reference/cr_3560/cli3.html#marker-2357437