Rob van Ooijen has sent me a really interesting question:
I've configured a switch port to be unconditionally a trunk with the switchport mode trunk configuration command. However, when the interface was enabled, I've got a dynamic trap indicating the trunking was still dynamic (and the show commands also showed negotiation of trunking is enabled).
In fact, adjacent layer-2 devices can negotiate a number of things these days, among them:
- Speed and duplex settings (we'll leave this can of worms safely closed);
- Trunking (a port can be access or trunk port);
- Trunk encapsulation (ISL or 802.1q);
With the switchport mode trunk command you just disable the trunking negotiation (the port will never become an access port), but the switch still runs Dynamic Trunking Protocol (DTP) on it, unless you disable DTP with switchport nonegotiate, which (in combination with all the other configuration commands) makes the port configuration totally static. More configuration can be found in Cisco's documentation and in the Basics of DTP document.
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