When using ISL, which VLAN are DTP frames sent?

When using Inter-Switch Link (ISL) encapsulation, Dynamic Trunking Protocol (DTP) frames are sent using VLAN 1. VLAN 1 is the default VLAN for Cisco switches and is typically used for various management operations, including DTP.

DTP is responsible for negotiating trunking between switches, and since it operates from the default VLAN, it utilizes VLAN 1 to transmit its control information. This is particularly important in environments where switches need to dynamically form trunk links and requires consistent communication across the default VLAN, which is VLAN 1.

In this context, while other VLANs may be configured for different traffic types within a network, DTP operations will always apply to VLAN 1 unless specifically altered, which is rare. Understanding this principle is critical for effectively managing Cisco switches and ensuring successful VLAN management across trunk links.