What is router on a stick and for what we use it ?

It is a setup when a switch is connected with multiple VLANs and router performs all the routing between the VLANs

it is used to connect more than one vlans using ethernet link as a trunk link, so imagne it like this you have many vlans all all are going through one router by bone link, that is a trunk link.

"Router on a Stick" is basically a setup of Router and a Switch connected via one Ethernet connection and configured 802.1q protocol. In this type of Setup, the Switch is configured with Multiple VLANs wherein each VLAN practically works as a port on the connected Router, as thus with different broadcast domains. Moreover, the routing is provided by the Router itself, as virtually, these VLANs are ports on the Router.

A subinterface is a logical interface which is created by dividing one physical interface into multiple interfaces. However, the Parent Physical Interface is used for data transmission. Sub-Interface can be used for variety of Purpose, Inter-Vlan Routing, Frame Relay and ATM. For Large number of VLANs, Layer3 Switch is preferred in comparison to Router on a stick configuration due to Switch Backplane Bandwidth.

Basic Cisco theory states that for hosts in different VLANs to communicate, a Layer 3 device must be involved to handle the routing between the VLANs. That device is a router, and there are special considerations that must be taken into account for both the physical router itself and the configuration you'll be writing. The router will be connected to a switch via a FastEthernet port (or higher). The router port cannot be a regular Ethernet port, since the router port will need the ability to send and receive data at the same time. The configuration of the interface is where things get interesting. Let's say we have two VLANs that will be using router-on-a-stick to communicate. Here is the VLAN information: VLAN 20: 20.20.20.0 /24 VLAN 40: 40.40.40.0 /24 The port on the switch that will be connected to the router's FastEthernet port must be in trunking mode, and you must know the trunking protocol in use. We'll go with the Cisco-proprietary ISL here. The physical FE port on the router will not have an IP address. The use of router-on-a-stick mandates the use of logical subinterfaces. While we don't have to use the VLAN numbers for the subinterface numbers, I've found this helps you keep the interfaces straight. One subinterface must be given an IP address in VLAN 20, and the other will have an IP address in VLAN 40. After creating subinterfaces fast 0.20 and fast 0.40, the config looks like this: interface fastethernet0    no ip address interface FastEthernet 0.20    ip address 20.20.20.1255.255.255.0 interface FastEthernet 0.40    ip address 40.40.40.1255.255.255.0 Believe it or not, you're almost done! Now we need the encapsulation statement under each subinterface. The subinterface statement must reflect both the VLAN number and the encapsulation type being used. When we're finished, the config would look like this: interface fastethernet0    no ip address interface FastEthernet 0.20    ip address 20.20.20.1255.255.255.0    encapsulation isl 20 interface FastEthernet 0.40    ip address 40.40.40.1255.255.255.0    encapsulation isl 40 And that's it! Your hosts in VLAN 20 should now be able to communicate with hosts in VLAN 40, and vice versa. A couple of final troubleshooting points - the most common error with router-on-a-stick is to put the wrong vlan number in the encapsulation statement. Also, make sure you have configured the router's IP address in VLAN 20 as the default gateway for hosts in VLAN 20, and do the same for VLAN 40.

Router on a stick is used when inter-VLAN routing is needed when you have an L2 switch and a router.

Router on a stick is a term used to describe the setup between switch and router using one ethernet link configured as an 802.1q trunk link.The switch configured with multiple VLAN's and router performs all routing between the different Networks(VLAN's).

Router-on-a-stick is a term frequently used to describe a setup up of connecting router and switch using ethernet link. In this setup, the switch is configured with multiple VLANs and the router performs all routing between the different networks/VLANs.

When router-on-a-stick is configured, the IP address assigned to the router's subinterfaces should be the default gateway setting on the hosts.

It is use communicate two different VLAN with help of  Configuring Sub Interface on Router.

used to describe a setup up that consists of a router and switch connected using one Ethernet link configured as an 802.1q trunk link. In this setup, the switch is configured with multiple VLANs and the router performs all routing between the different networks/VLANs.