Implement Trunking." When connecting Cisco switches to non-Cisco devices, the choice is 802.1Q. Remember, there are no restrictions regarding protocol choice on a switch that sup- ports 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 Chapter 5: Trunking 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 con- nected using ISL elsewhere. Trunk Negotiation Some Cisco switches support the Dynamic Trunking Protocol (DTP). This protocol attempts to determine what 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. 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 SW1 (config-if)# switchport mode trunk SW1 (config-if)# switchport nonegotiate Figure 5-4 shows the possible switchport modes. Remember, not all switches sup- port all modes. A port can be set to the mode access, which means it will never be a trunk; dynamic, which means the port may become a trunk; or trunk, which means the port will be a trunk regardless of any other settings. Description Remote side must be this mode for port to become trunk Prevents the port from becoming a trunk even if the other side access Doesn't matter of the link is configured as a trunk. dynamic desirable Port will actively attempt to convert the link to a trunk. trunk, desirable, dynamic auto Port will become a trunk if the other side is configured to be a trunk. Port will not actively attempt to convert a link