Configuring a trunk involves determining which port will be a trunk, which protocol the trunk will run, and whether and how the port will negotiate. Optionally, you may also wish to limit which VLANs are allowed on the trunk link.
The Cisco 3750 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 3750 switch.
You might think the first step is 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 (
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:
switchport trunk encapsulation dot1q3750-IOS(config-if)#
switchport mode trunk
Should you wish to subsequently remove trunking from the
interface, you’d use the command
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 ...