Routing Between VLANs
Routing is the process of determining where to send data packets destined for addresses
outside of the local network. Routers gather and maintain routing information to enable the
transmission and receipt of data packets. For traf?¬?c to cross from one VLAN to another, a
Layer 3 process is necessary.
This section describes the operation of inter-VLAN routing using a router on a stick.
Understanding Inter-VLAN Routing
Inter-VLAN communication occurs between broadcast domains via a Layer 3 device. In a
VLAN environment, frames are switched only between ports within the same broadcast
domain. VLANs perform network partitioning and traf?¬?c separation at Layer 2. Inter-
VLAN communication cannot occur without a Layer 3 device, such as a router. Use IEEE
802.1Q to enable trunking on a router subinterface.
Example: Router on a Stick
Figure 2-31 illustrates a router attached to a core switch. The con?¬?guration between a router
and a core switch is sometimes referred to as a router on a stick.
Figure 2-31 Router on a Stick
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.
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