ntervention to gain access to the network. Both approaches have merit, and they can even be used together. Figure 5-4 shows the possible switchport modes. Remember, not all switches support all modes. A port can be set to the mode access, which means it will never be a trunk; dynamic, which means the port may become a trunk; or trunk, which means the port will be a trunk regardless of any other settings. Description Remote side must be this mode for port to become trunk Prevents the port from becoming a trunk even if the other side access Doesn't matter of the link is configured as a trunk. dynamic desirable Port will actively attempt to convert the link to a trunk. trunk, desirable, dynamic auto Port will become a trunk if the other side is configured to be a trunk. Port will not actively attempt to convert a link 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 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 trunk. How Trunks Work 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 hard- coded as a trunk or set to dynamic desirable. Configuring Trunks 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 