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Trunk Ports are switchports on Cisco switches that interconnect switches. The trunk port carries traffic for all the VLans across the switches it interconnects. switchport can be enabled and configured as trunks without much trouble.

The following procedure helps setting a switchport into a Trunk port with 802.1q encapsulation.

Enter the Interface Configuration mode

To configure the port enter the Interface config mode from the global configuration mode.

ciscoswitch# conf t
ciscoswitch(config)# interface gigabitethernet 1/1

Set the Switchport mode as Trunk

Once in the configuration mode for the selected interface (here GigabitEthernet 1/1), make the port as switchport and set the switchport mode as trunk and then set trunk port encapsulation.

ciscoswitch(config-if)# switchport
ciscoswitch(config-if)# switchport mode trunk
ciscoswitch(config-if)# switchport trunk encapsulation dot1q

This turns the switchport as a Trunk port.

Optional recommended configurations

Enable UDLD

UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. When UDLD detects a unidirectional link, it administratively shuts down the affected port and alerts you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.

ciscoswitch(config-if)# udld port aggressive

Setting the udld port mode as aggressive, it can detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected ports on fiber-optic links.

Enable Logging

Being a trunk port, it is advisable to enable logging on the switchport link status

ciscoswitch(config-if)# logging event link-status
ciscoswitch(config-if)# logging event bundle-status
ciscoswitch(config-if)# logging event trunk-status

To see the port configuration

ciscoswitch# sh run interface gi1/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 logging event link-status
 logging event bundle-status
 logging event trunk-status
 udld port aggressive

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