A vlan configuration

A vlan configuration on a router is slightly different than on a switch.  On a switch, you would create the vlan and then the routed vlan interface.

On a router you tie a vlan to a subinterface.  You need to create the subinterface and then do the encapsulation dot1q <vlan ID> command.  This command basically assigned the vlan to the subinterface.  In this example, there are 2 vlans, vlan 100 and 201.  On the eth 0/0 interface, I created two subinterfaces.  fastethernet0/0.100 and f0/0.201.  Notice that I matched the subinterface number with the vlan ID.  You do not have to do this, but it is a best practice and really helps you as the administrator keep things organized.  So, under f0/0.100, there is the encapsulation dot1q 100, which basically binds vlan 100 to subinterface f0/0.100, then I assigned the IP.  The same is for the other subinterface.  So the router is trunking two vlans on its f0/0 interface, vlan's 100 and 201.

interface FastEthernet0/0
no ip address
speed 100
full-duplex
!
interface FastEthernet0/0.100
description test Network
encapsulation dot1Q 100
ip address 192.168.102.1 255.255.255.128
no snmp trap link-status
!
interface FastEthernet0/0.201
description  Office
encapsulation dot1Q 201
ip address 205.127.102.129 255.255.255.128
no snmp trap link-status