Octet

Say this first

DTP is Cisco's protocol for negotiating whether a link becomes a trunk. access never trunks. trunk always trunks and still sends DTP. dynamic desirable actively asks to trunk. dynamic auto trunks only if the other side asks. switchport nonegotiate stops DTP frames. Auto plus auto stays access. Most current Catalyst switches default to dynamic auto, so I set every port to access or trunk explicitly and add nonegotiate on trunks.

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Result for each pair

	access	trunk	desirable	auto
access	access	mismatch	access	access
trunk	mismatch	trunk	trunk	trunk
desirable	access	trunk	trunk	trunk
auto	access	trunk	trunk	access

"Mismatch" means one side tags frames and the other does not: only untagged traffic works, and STP may flag inconsistencies. A side set to trunk with nonegotiate against a dynamic side gives the same mismatch, because the dynamic side never hears DTP.

Details interviewers like

Dynamic trunking fails when the two switches have different VTP domain names; the switch logs %DTP-5-DOMAINMISMATCH.
In show interfaces trunk, an encapsulation of n-802.1q means the trunk was negotiated.
Older platforms such as the Catalyst 2950 and 3550 defaulted to dynamic desirable.
NX-OS and most other vendors do not run DTP. Against them, set the Cisco side to trunk.

Output

SW1# show interfaces GigabitEthernet1/0/24 switchport | include Administrative Mode|Operational Mode
Administrative Mode: dynamic auto
Operational Mode: static access

They will ask next

Does switchport mode trunk stop DTP?
No. The port still sends DTP frames so the other side can follow. Only switchport nonegotiate (or access mode) stops them.
Why does a trunk to a router or a server need switchport mode trunk?
They do not speak DTP. A dynamic auto port facing them never becomes a trunk.

Traps

Assuming switchport mode trunk alone disables DTP.
Leaving user ports in dynamic auto. That is exactly what switch spoofing needs.

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