What encapsulation type is favored in DTP trunk encapsulation negotiation?

In the context of Dynamic Trunking Protocol (DTP) trunk encapsulation negotiation, the favored encapsulation type is ISL, which stands for Inter-Switch Link. ISL is a Cisco proprietary protocol that encapsulates Ethernet frames with VLAN information, enabling the identification of VLANs across trunk links. This allows switches to maintain VLAN information as traffic passes over the trunked link.

DTP allows switches to negotiate whether a trunk link will be established automatically and the encapsulation type to be used. While both ISL and 802.1Q are used for trunking, when DTP is in use, it generally defaults to ISL if it is available on the switch interface. 802.1Q is the industry standard for VLAN tagging, but when negotiating via DTP, ISL takes precedence on Cisco devices primarily due to its compatibility features within Cisco networks.

By understanding the nuances and proprietary nature of ISL, you can see why it is favored during DTP negotiations, as it provides greater functionality for trunking in Cisco environments, particularly when legacy support is a consideration.