 ends up happening is that whoever installs the trunk uses whichever protocol he is used to. Cisco has recommendations on how to set up trunks between Cisco switches. This document (Cisco document ID 24067) is titled "System Requirements to Implement Trunking." When you're connecting Cisco switches to non-Cisco devices, the choice is 802.1Q. Remember, there are no restrictions regarding protocol choice on a switch that supports both. If you need to connect a 3Com switch to your Cisco network, you can do SO with an 802.1Q trunk even if your Cisco network uses ISL trunks elsewhere. The trunking protocol is local to each individual trunk. If you connect to a 3Com switch using 802.1Q, the VLANs on that switch will still be accessible on switches connected using ISL elsewhere. The Cisco Nexus line, which appears to embrace open standards more than previous devices, does not support ISL. Trunk Negotiation Some Cisco switches support the Dynamic Trunking Protocol (DTP). This protocol attempts to determine which trunking protocols are supported on each side and to establish a trunk, if possible. Trunk negotiation includes the VLAN Trunking Protocol (VTP) domain name in the process. For DTP to successfully negotiate, both switches must have the same VTP domain name. See Chapter 6 for details on configuring VTP domains. Chapter 5: Trunking An interface running DTP sends frames every 30 seconds in an attempt to negotiate a trunk. If a port has been manually set to either "trunk" or "prevent trunking," DTP is unnecessary and can be disabled. The IOS command switchport nonegotiate disables DTP: 3750-1 (config-if)#switchport mode trunk 3750-1 os(config-if)#switchport nonegotiate Many networking engineers prefer to issue the switchport nonegoti ate command to all ports as a security precaution. If someone were to attach a rogue switch to the network and manage to negotiate a trunk, she would potentially have access to every VLAN on the switch. I prefer to disable all ports by default SO that any attached device requires human i