Communication between VLANS.
In a layer 2 switch, the communication between VLAN's is not possible without the help of a layer 3 device,which is a router.If a computer wants to send data frames to a computer which is in another VLAN, then these two VLANs should be connected to a router.The communication between VLANs can only happen through routing.This method of using a router to send data frames from a device in one VLAN to a device in another VLAN is called inter VLAN routing.
In the above figure the three PC's PC-1,PC-2 and PC-3 are in VLAN1,VLAN2 and VLAN3. The switchport F0/1 connected to the router is configured as the trunk port. Instead of using a seperate router for inter VLAN routing , we can use a Layer 3 switch(also called as multilayer switch).
We can use layer 3 switch as a layer 2 switch or as a router. we can use it as a router to perform interVLAN routing.We can configure the interfaces of layer3 switch to work as a switch port(layer 2) or as a router interface by assigning an ip address to it.We can configure the SVI(Switched Virtual Interface) and assign the IP address to it. If we want ot use any switchport as a layer 2 port(for switching) then we have to issue the command "switchport" in the interface configuration mode as shown below.
Switch(config)# interface
Switch(config-if)#switchport
Switch(config)# interface
Switch(config-if)#no switchport
Switch(config)# interface
Switch(config-if)#switchport
Switch(config)# interface
Switch(config-if)#no switchport