Supporting Multiple VLANs on a Single Trunk
How to configure trunking

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.