DTP is a means for switch ports to automatically form and negotiate trunks. Another feature that you don’t see used very often in large enterprise networks, but it still needs to be covered.
If you buy two new 2960s from Cisco and plug them together with a patch cable they will automatically form a trunk. How does this happen? Because the default port setting on all interfaces for a new switch is …
switchport mode dynamic desirable
Here’s a table that shows the different DTP configuration options…
Being a Cisco Protocol, DTP will first try to negotiate ISL. If it can’t it will then fall back to dot1q. It’s pretty straight forward so Im not going to beat the horse but here are a couple of other quick notes on DTP.
-You can disable DTP entirely with the ‘switchport nonnegotiate’ command. This is very common to see on hard coded trunk ports
-Ports that are configured in access mode (switchport mode access) have DTP disabled. There is no need to use the ‘switchport nonnegotiate’ command on access ports. Note: I previosuly had this wrong based on info from the CCIE R&S fourth edition cert guide. The chart on page 53 clearly states that access ports will still send DTP unless you disable it with the nonnegotiate command. Thanks to a reader for catching this!