The Cisco Catalyst 5000 series switches have long been a staple of many server rooms and wiring closets in the corporate world. Although originally only a Layer 2 switch, one method that can be used to add routing functionality to these models is the addition of a Route Switch Module (RSM). An RSM is essentially a Cisco router on an add-in module that plugs directly into the backplane of a Catalyst 5000 switch. Just like a Cisco router, an RSM runs Cisco’s IOS software, and is configured in a very similar manner.
The main benefit of using an RSM is that it eliminates the need for an external router, thus speeding up the routing process. However, an RSM is still a router in the traditional sense. Although the fact that packets do not need to be passed to an external device makes the process a little faster, routing is still occurring using the IOS software, using traditional routing methods. As such, a Catalyst 5000 with an RSM installed can still be considered a Layer 3 switch, but will not provide the dramatic increases in speed that other methods listed here do. More than anything, the RSM provides convenience, adding routing capabilities to a Layer 2 switch.