Switch ports are configured for trunking using Cisco IOS commands.
To configure a switch port as an ISL or 802.1Q trunking port, follow these
steps on each trunk interface:

Step 1 Enter interface configuration mode.
Step 2 Shut down the interface to prevent the possibility of
premature auto configuration taking place.
Step
3 Select the trunking encapsulation. Note some switches support only ISL or
802.1Q.
Step 4 Configure the interface as a
Layer 2 trunk.
Step 5 Configure the trunking
native VLAN number for 802.1Q links. This number must match at both ends of an
802.1Q trunk.
Step 6 Configure the allowable
VLANs for this trunk. This is necessary if VLANs are restricted to certain
trunk links. This is best practice with the Enterprise Composite Model and
leads to the correct operation of VLAN interfaces.
Step 7 Use the no shutdown command on
the interface to activate the trunking process.
Step
8 Verify the trunk configuration using show
commands.