In some network environments,
VLANs are associated with individual networks or subnetworks. In an IP network,
each subnetwork is mapped to an individual VLAN. Configuring VLANs helps
control the size of the broadcast domain and keeps local traffic local.
However, network devices in different VLANs cannot communicate with one another
without a Layer 3 device (router) to route traffic between the VLAN, referred
to as inter-VLAN routing. You configure one or more routers to route traffic to
the appropriate destination VLAN.
Figure 1. Routing Topology
Example. This figure shows
a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20.
The router has an interface in each VLAN.
When Host A in VLAN 10 needs
to communicate with Host B in VLAN 10, it sends a packet addressed to that
host. Switch A forwards the packet directly to Host B, without sending it to
When Host A sends a packet to
Host C in VLAN 20, Switch A forwards the packet to the router, which receives
the traffic on the VLAN 10 interface. The router checks the routing table,
finds the correct outgoing interface, and forwards the packet on the VLAN 20
interface to Switch B. Switch B receives the packet and forwards it to Host C.
When static routing is
enabled on Switch A and B, the router device is no longer needed to route