Here is the part you want to see - Troubleshooting! Question: Do trunks form between SW1 / SW2 and SW2 / S3? Remember: VLAN trunks are the highway that every bit of information can pass upon, no matter what VLAN it came from. When the VLAN frame gets closer to its destination, it will travel on roads only avaliable for that VLAN. The packets for SW1/2 so far show DTP and PAgP. Now, PAgP is Cisco-proprietary Etherchannel, but DTP is for trunking negotiation between switches. Wait!