Configuring a trunk involves determining what port will be a trunk, what protocol the trunk will run, and whether and how the port will negotiate. Optionally, you may also wish to limit what VLANs are allowed on the trunk link.
The Cisco 3550 is an excellent example of an IOS switch. This section will walk you through configuring one of the Gigabit ports to be an 802.1Q trunk using a 3550 switch.
You might think that the first thing to do would be to specify that the port is a trunk, but as you're about to see, that's not the case:
switchport mode trunkCommand rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
On an IOS switch capable of both ISL and 802.1Q, you must specify a trunk
encapsulation before you can configure a port as a trunk. (
trunk encapsulation is an unfortunate choice for the command
because, as you now know, 802.1Q does not encapsulate frames like ISL does. Still, you
must follow Cisco's syntax.) Once you've chosen a trunking protocol, you are free to
declare the port a trunk:
interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport mode trunk
Should you wish to subsequently remove trunking from the interface, the command to
do so is
switchport mode access.
By default, all VLANs on a switch are included in a trunk. But you may have 40 VLANs, and only need to trunk 3 of them. Because broadcasts from all allowed VLANs will be sent on every trunk port, excluding ...