2.3. Inter-VLAN Routing
In most networks today, switches support multiple VLANs. But if those switches have no Layer 3 capabilities, then packets cannot travel between VLANs. Without Layer 3 capabilities, the switch or switches must be connected to an external router. The best and most efficient way to set this up employs a single trunk link between the switch and the routing device. That way, the trunk can carry the traffic for multiple VLANs. When the traffic reaches the router, either a router or multilayer switch, it can then be routed. This single physical link, the trunk, must be Fast Ethernet or greater to support Inter-Switch Link (ISL) encapsulation. The 802.1Q trunking protocol is supported on 10 Mb Ethernet router interfaces.