Supporting Multiple VLANs on a Single Trunk
Configuring an ISL trunk

In the example, interface Fastethernet 2/1 has been configured as a trunk link for ISL that is permanently on. DTP negotiation is not allowed. The trunk link will carry VLAN traffic for VLANs 1-5 and 1002-1005. VLAN 2-5 are configured on various access ports on the switch and the trunk links need to carry the frames for these VLANs as well as the frames for the system VLANs 1, 1002-1005.  – 

NOTE:

It is best practice is to shut down an interface while configuring trunking attributes so that premature autonegotiation cannot occur.

Configuring a Port for ISL Trunking with No DTP
When configuring the Layer 2 trunk to not use DTP, the following syntax is used so that the trunk mode is set to on and NO DTP messages are sent on the interface:

  • First, enter the shutdown command in the interface mode.
  • Enter the switchport trunk encapsulation command.
  • Enter the switchport mode trunk command.
  • Enter the switchport trunk native vlan vlan_id command.
  • Enter the switchport nonegotiate command.
  • Finally, enter the no shutdown command.

This example shows how to configure a port for ISL trunking with no DTP:

Switch#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)#interface fastethernet 2/1
Switch(config-if)#shutdown
Switch(config-if)#switchport trunk encapsulation isl
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport trunk native vlan 99
Switch(config-if)#switchport nonegotiate
Switch(config-if)#no shutdown
Switch(config-if)#end
Switch#exit

Verifying the ISL Trunk Configuration
Use show commands to display port information, switch port information, or trunking information.