First of all, telnet to your switch, these instructions are for Cisco switches, that’s all I ever worked with anyways, heh. All those instructions use IOS.

First of all you need to pick the port that you’ll be working on, make sure you have physical access to the server you’re going to be working with, I’m not responsible for some settings going poof and you having to drive down to your data center to fix the issue.

So, the interface that I will be working with is FastEthernet1/8 — Enter configuration mode and go to the port

conf t
int FastEthernet1/8

Enable trunking and 801q capsulation

switchport mode trunk
switchport trunk encapsulation dot1Q
switchport trunk native vlan 1000

Make sure the native VLAN is the same VLAN that the network was running on previously, read more information why here:

To establish 802.1q trunking both sides must be in the same native VLAN this is because the encapsulation is not setup yet and the 2 switches must talk over an un-encapsulated link (native VLAN) to setup the encapsulation in the first place. Why this works is because neither side is encapsulating packets with its VLAN tag since they are both talking over their native VLAN, basicly neither side knows that the other side is in a different VLAN to begin with and they are just sending unencapsulated packets back and forth. So if you set a port on the core switch as native vlan 5 for example and connected a dumb switch to it vlan 5 traffic would go un encapsulated to the dumb switch and it can understand it but it will put it in its VLAN 1 ports there is no actual trunking going on. No 802.1q or ISL!
juniperr @ DevShed