Reasons for Layer 3 Switching

Here is a short paper that may justify using layer three switches on a network. I hope that it is informative.


Reasons for L3 switching

Layer 3 switches can be used to route internal LAN or inter-VLAN traffic thereby reducing the amount of traffic processed by an enterprise router. This is especially effective when a L3 switch is used to connect a backbone within an enterprise network. However, the L3 switch will still have to rely on the network router to learn routing tables and route packets that have to be sent over the WAN. The enterprise router will mostly serve as the network gateway; thus, it will generally be used to perform inter-Autonomous System (AS) routing.


VLANs and Cisco’s Three Layered Hierarchical Model Design

VLANs are virtual LANs created by an administrator for various reasons. VLANs will behave as if devices (within the same VLAN) are attached to the same switch and media. They will also create their own broadcast domain(s). Inter-VLAN traffic must be routed through a L3 device-this could be a router or a L3 switch. Often times L3 switches are used on VLANed networks to reduce the amount of processing a router has to perform when routing VLAN traffic. This is in accordance to Cisco’s Three Layered Hierarchical Model Design which states that the Core layer (most often the enterprise router(s)) should be used to specifically move traffic as quickly and efficiently as possible. See the following website for more information about the Three Layered Hierarchical Model Design: http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2002.htm.


Routing-on-a-Stick

A L3 switch could be omitted from a VLANed network with “routing-on-a-stick.” This network design uses the network router to do “trunking” between VLANs thereby managing inter-VLAN traffic. Configuring “routing-on-a-stick” is pretty straight-forward. A port on the router has to be configured to do trunking, and sub-interfaces must be configured on a router port. However, routing-on-a-stick does add to the processing the router is already performing on the network. In addition, for some network administrators, routing-on-a-stick may add to the complexity of a router configuration. Usually this setup is not a problem on small/simple networks, but could become cumbersome and difficult on larger, complex networks.


Conclusion

In conclusion, according to the Cisco Three Layered Hierarchical Design Model, if a router is acting strictly as a core layer device the only thing it should do is quickly move traffic. Yet, a “collapsed network” could be used where a router will act as both a core and distribution layer device. In the case of VLANs, the router will have trunking configured on one of its ports thereby adding to the processing that the router must do on the network while routing inter-VLAN traffic. Although the collapsed network design is less expensive since it eliminates the need for additional equipment (i.e. the L3 switch) it is not always the most efficient setup for a network using multiple VLANs.

Cheers,
y0duh