Allow VLAN 20 on the trunk link.Refer to curriculum topic: 6.2.2Enabling DTP on both switches simply allows negotiation of trunking. The Negotiation of Trunking line in the graphic shows that DTP is already enabled. The graphic also shows how the native VLAN is 1, and the default VLAN for any Cisco switch is 1. The graphic shows the PCs are to be in VLAN 20. Explanation: The problem is that the switch SW1 is not configured correctly. It should be configured to allow traffic from VLAN 20 to pass through.