 to a trunk. trunk, desirable trunk Port is a trunk regardless of the other side. Doesn't matter Figure 5-4. Possible switch port modes related to trunking How Trunks Work The two dynamic modes, desirable and auto, refer to the method in which DTP will operate on the port. desirable indicates that the port will initiate negotiations and try to make the link a trunk. auto indicates that the port will listen for DTP but will not actively attempt to become a port. The default mode for most Cisco switches is dynamic auto. A port in this condition will automatically become a trunk should the remote switch port connecting to it be hardcoded as a trunk or set to dynamic desirable. 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. 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-I0S(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 encap- sulation 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. Bu