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.