Just like
subnetworks, (remember subnetworks = VLANs) a router is needed to
route information between different VLANs. There
are several different ways to do this with the router.
One way is to have a router with a
separate Ethernet interface for every VLAN (i.e. subnetwork). Figure
shows the router with two Ethernet interfaces. The Ethernet 0
interface is configured with an IP address of 172.30.1.1/24 and will
be a member of VLAN 1 on the switch, while the Ethernet 1 interface is
configured with an IP address of 172.30.2.1/24 and will be a member of
VLAN 2 on the switch.
Figure
shows how the switch and router interconnect.
If information is needed to be sent
from a host on one VLAN (subnetwork) to a host on another VLAN (subnetwork),
these packets will need to be sent to and routed through the router. 
Note:
The router must be running a routing protocol to route between the two
subnetworks.
Example:
Router(config)# router rip
Router(config-router)# network 172.30.0.0
One of the disadvantages to this type
of router configuration is that the router must have a separate
Ethernet interface for every VLAN (subnetwork). This may not scale
well for networks with a lot of different VLANs. One solution to this
problem is to use “Router-on-stick” or “One-Armed-Router”
(OAR).
With the Router-on-stick method, only
one physical Ethernet interface is used. The physical interface is
divided into sub-interfaces, one for each VLANs (subnetworks). (Note:
You may use secondary addresses instead of sub-interfaces to
accomplish this, but secondary addresses will eventually be no longer
supported in future versions of the Cisco IOS.) 
Notice that with the use of
Router-on-a-stick, that only one physical interface is need. The
disadvantage to this is that this single link between the router and
the switch will be used for all VLAN traffic to and from the switch.
The port on the switch which is connected to the router’s Ethernet
interface will also need to be capable of doing “trunking,” either
with Cisco’s proprietary ISL (InterSwitch Link) or IEEE 802.1Q.
Here is the router configuration (also
shown in Figure ):
Router# config t
Router(config)# interface ethernet 0.1
Router(config-subif)# ip address 172.30.1.1 255.255.255.0
Router(config-subif)# encapsulation isl 1 {1 = VLAN 1}
Router(config-subif)# no ip redirects {recommended}
Router(config)# interface ethernet 0.2
Router(config-subif)# ip address 172.30.2.1 255.255.255.0
Router(config-subif)# encapsulation isl 2 {2 = VLAN 2}
Router(config-subif)# no ip redirects {recommended}
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