Each VLAN must have a separate connection on the router, and you
must enable 802.1Q trunking on those connections. The router already knows about
directly connected networks. The router must learn routes to networks to which it is not
directly connected.
10.1.1.2
VLAN 1 VLAN 2
802.1Q
Router on
a Stick
10.2.2.2
Routing Between VLANs 65
To support 802.1Q trunking, you must subdivide the physical FastEthernet interface of the
router into multiple, logical, addressable interfaces, one per VLAN. The resulting logical
interfaces are called subinterfaces. This is illustrated in Figure 2-32.
Figure 2-32 Subinterfaces
Without this subdivision, you would have to dedicate a separate physical interface to each
VLAN.
Example: Subinterfaces
In the ?¬?gure, the FastEthernet 0/0 interface is divided into multiple subinterfaces:
FastEthernet 0/0.1, FastEthernet 0/0.2, and FastEthernet 0/0.3.
Configuring Inter-VLAN Routing
To be able to route between VLANs on a switch, you will need to be able to con?¬?gure inter-
VLAN routing.
In Figure 2-33, the FastEthernet 0/0 interface is divided into multiple subinterfaces:
FastEthernet 0/0.1 and FastEthernet 0/0.
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