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Configure Router on a Stick

A while ago I talked about putting different VLANs on a switch, remember a VLAN is virtual network that although physically it may look like on the same network that does not always mean the case. By having VLANs you are segmenting the network and the only way to get to the other side is having a router. I have already configured the Cisco switch as posted in Creating VLANs but in summary I have three VLANs total VLAN 1 which is the native VLAN, VLAN 2 and VLAN 3 (which is called support. If you like to understand how create VLANs on a switch follow the post above. If you look at the network topology below you can see where Cisco came up with the name "Router on Stick" each PC is on its own network and needs the router in order for traffic to pass between the networks. Like before I have three VLANs total. VLAN 1 which is the native VLAN, VLAN 2 and VLAN 3 (which is called "Support").

You have to create sub-interfaces on the router to route between different VLANs these sub-interfaces do not correspond to the VLANs so you could put any number but for manageability usually people use the same sub-interface has the VLAN ID. To create a sub-interface start typing the interface that will be physically connected to the switch at the end add a period and number.

Router#config t 
Enter configuration commands, one per line. End with CNTL/Z. 
Router(config)#interface fastEthernet 0/0.2
Router(config-subif)#
You are now in the sub-interface configuration, we want to tell the router which VLAN this interface will be a part of. In this example I am typing "encapsulation dot1Q 2" dot1Q is an IEEE standard 802.1Q , the number "two" is the VLAN this interface will be a part of. Just like a real interface this interface must have an IP address for end devices to reach it. In this example we are using 192.168.2.1 with a Class C subnet mask. (i.e 255.255.255.0) 
Router(config-subif)#encapsulation dot1Q 2
Router(config-subif)#ip address 192.168.2.1 255.255.255.0
Router(config-subif)#no shut
We do the same for VLAN 3 (Support) in which we will create a new sub-interface in this example I am using sub-interface 3, and I'm going to be putting this sub-interface in VLAN 3 by using "encapsulation dot1Q 3". Remember the sub-interface number has no effect on which VLAN this interface is a part of. I'm also adding a IP address of 192.168.3.1 with a Class C subnet mask. 
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface fastEthernet 0/0.3
Router(config-subif)# 
Router(config-subif)#encapsulation dot1Q 3
Router(config-subif)#ip address 192.168.3.1 255.255.255.0
Router(config-subif)#no shut
The last VLAN is the native VLAN and by default is VLAN 1, for this configuration go to the physical interface and add an IP address for this example we are using 192.168.1.1 with a Class C subnet mask. 
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface fastEthernet 0/0
Router(config-if)#ip address 192.168.1.1 255.255.255.0
Router(config-if)#no shut
The last thing we have to setup is on the Cisco Switch in which we have to specify which interface on the switch will pass all the VLANs, this is known as the trunking port. Go into configuration mode and for this example we selected FastEthernet0/1 apply the "switchport mode trunk" command and since we are using the default native VLAN which is not always recommended that's all the configuration needed. We now have three different networks separated by VLANs, if you issue a "show ip route" on the router we will see each sub-interface and route to each of them. All devices should be able to ping each other. 
Switch>en
Switch#config t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#interface fastEthernet 0/1
Switch(config-if)#switchport mode trunk

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

C    192.168.1.0/24 is directly connected, FastEthernet0/0
C    192.168.2.0/24 is directly connected, FastEthernet0/0.2
C    192.168.3.0/24 is directly connected, FastEthernet0/0.3

I hope this information is helpful I have also attached a PDF of the running-config of both devices to give you an example of the configuration. (Router-On-Stick-Example-RunConfig) If you have any questions post them below. Like always if you an idea of the next topic let me hear it.