16-17
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 16 Configuring VLANs
Configuring VLAN Trunks
• A port in dynamic mode can negotiate with its neighbor to become a trunk port. If you try to enable
IEEE 802.1X on a dynamic port, an error message appears, and IEEE 802.1X is not enabled. If you
try to change the mode of an IEEE 802.1X-enabled port to dynamic, the port mode is not changed.
• Protected ports are supported on IEEE 802.1Q trunks.
Configuring a Trunk Port
Beginning in privileged EXEC mode, follow these steps to configure a port as an IEEE 802.1Q
trunk port:
To return an interface to its default configuration, use the default interface interface-id interface
configuration command. To reset all trunking characteristics of a trunking interface to the defaults, use
the no switchport trunk interface configuration command. To disable trunking, use the switchport
mode access interface configuration command to configure the port as a static-access port.
This example shows how to configure a port as an IEEE 802.1Q trunk. The example assumes that the
neighbor interface is configured to support IEEE 802.1Q trunking.
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface
Switch(config-if)# switchport mode dynamic desirable
Switch(config-if)# end
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 interface interface-id Enter the interface configuration mode and the port to be configured for
trunking.
Step 3 switchport mode {dynamic {auto |
desirable} | trunk}
Configure the interface as a Layer 2 trunk (required only if the interface
is a Layer 2 access port or to specify the trunking mode).
• dynamic auto—Set the interface to a trunk link if the neighboring
interface is set to trunk or desirable mode.
• dynamic desirable—Set the interface to a trunk link if the
neighboring interface is set to trunk, desirable, or auto mode.
• trunk—Set the interface in permanent trunking mode and negotiate
to convert the link to a trunk link even if the neighboring interface is
not a trunk interface.
Step 4 switchport access vlan vlan-id (Optional) Specify the default VLAN, which is used if the interface stops
trunking.
Step 5 switchport trunk native vlan vlan-id Specify the native VLAN.
Step 6 end Return to privileged EXEC mode.
Step 7 show interfaces interface-id switchport Display the switchport configuration of the interface in the Administrative
Mode and the Administrative Trunking Encapsulation fields of the
display.
Step 8 show interfaces interface-id trunk Display the trunk configuration of the interface.
Step 9 copy running-config startup-config (Optional) Save your entries in the configuration file.