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Network Warrior by Gary A. Donahue

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Configuring Trunks

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.

IOS

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:

3550-IOS(config-if)#switchport mode trunk
Command 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

Tip

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 ...

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