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When it comes to the Catalyst switches, a key topic, and one that requires very careful reading of tasks, is the Dynamic Trunk Protocol (DTP). Let us review some key facts about this protocol and get our blog readers all set for the exam in this area.
First, a review of the modes, the commands that initiate them, and whether or not DTP frames are sent:
Notice how important committing this to memory is going to be. For trunk formation, we examine if DTP frames are sent from one of the ends of the link. For example – with AUTO-DESIRABLE we get a trunk since one side is willing to trunk and the other side is sending DTP frames.
Another very important point is how we eliminate DTP from the network trunk. We use switchport mode trunk in conjunction with switchport nonegotiate. This ensures unconditional trunking and eliminates the sending of the DTP frames. It is very important not to confuse this setting with DTP mode OFF (switchport mode access).
As stated earlier, reading very carefully on a task regarding this is critical. Also, at times there can be more than one acceptable configuration. For example, if SW1 needs to initiate a trunk to SW2, and that is the only requirement, you could have SW1 do trunk modes of ON or DESIRABLE and ensure SW2 is AUTO.
Best practices in the lab and in production typically dictate that we eliminate DTP whenever possible. In the lab, this would mean that there are no requirements stated that we use it. For example, if the task simply states to form a trunk between SW1 and SW2, we should consider:
switchport trunk encapsulation dot1q switchport mode trunk switchport no negotiate
Anthony Sequeira CCIE, CCSI
Twitter: @compsolv
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Tags: CCIE, CCIE R&S, CCIE R&S Training, CCIE Routing & Switching, CCIE Training
