Virtual LANs - Virtual LANs (2)

By creating and implementing VLANs in your switched network, you can break up broadcast domains at layer 2. For hosts on different VLANs to communicate, you must have a router or layer 3 switch.

You must have the same VTP domain name on all switches in order to share VLAN information between the switches. At least one of the switches must be a VTP server; the other switches should be set to VTP client.

The valid modes of a VLAN trunk on a switch are dynamic auto, dynamic desirable, trunk (on), and nonegotiate.

By default, all VLANs are allowed on the trunk link and you must remove by hand each VLAN that you don't want traversing the trunked link.

If you are on a 2950 switch, then the interface command is just switchport mode trunk, since the 2950 can only run the IEEE 802.1Q version. However, a 3550 can run both ISL and 802.1Q, so you must use the encapsulation command. The argument to choose 802.1Q for a trunking protocol is dot1q.

To set a switch port to trunk mode, which allows all VLAN information to pass down the link, use the switchport mode trunk command.

All Cisco switches are VTP servers by default. No other VTP information is configured on a Cisco switch by default. You must set the VTP domain name on all switches to be the same domain name or they will not share the VTP database.

Virtual Trunk Protocol (VTP) is used to pass a VLAN database to any or all switches in the switched network. The three VTP modes are server, client, and transparent.