Describing Routing Between VLANs
Inter-VLAN routing using an external router and a single trunk

If a switch supports multiple VLANs but has no Layer 3 capability to route packets between those VLANs, the switch must be connected to a router external to the switch. This will be most efficiently accomplished by providing a single trunk link between the switch and the router that can carry the traffic of multiple VLANs, which can in turn be routed, by the router. This single physical link must be FastEthernet or greater to support subinterfaces.

In Figure , the clients on VLAN10 need to establish sessions with a server that is in VLAN 20. This will require that traffic be routed between the VLANs as described in Figure .

In the figure, the router can receive packets on one VLAN and forward them to another VLAN. To perform inter-VLAN routing functions, the router must know how to reach all VLANs being interconnected. The router must have a separate logical connection (subinterface) for each VLAN running between the switch and the router and ISL, or 802.1Q trunking must be enabled on the single physical connection between the router and switch. The routing table will show as directly connected to all the subnets associate with the VLANs configured on the router subinterfaces. The router must learn routes to networks not configured on directly connected interfaces through dynamic routing protocols.

External Router with Single Interface: Advantages and Disadvantages
Figure describes the advantages and disadvantages of using an external router for inter-VLAN routing.