When you configure command "switchport trunk encapsulation negotiate" the interface negotiate with the neighboring interface to become an ISL (preferred) or 802.1Q trunk, depending on the configuration and capabilities of the neighboring interface.
So if you have a neighboring switch like 2950 who only supports dot1q then DTP will negotiate to dot1q but if your neighbor switch supports both ISL and Dot1q and you have not configured manually something on the neighbor switch then ISL is preferred.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...