Chapter 7: Multilayer Switching (MLS)

The CCNP Exam Topics Covered in This Chapter Include the Following:

• Identify the components necessary to effect multilayer switching

• Apply flow masks to influence the type of MLS cache

• Describe layers 2, 3, 4 and multilayer switching

• Verify existing flow entries in the MLS cache

• Describe how MLS functions on a switch

• Configure a switch to participate in multilayer switching

• Determine appropriate multilayer switching architectures for specific needs

The expression Multilayer Switching (MLS) can be very confusing. If you ask 10 different vendors what it means, you will probably get 11 different answers! After all, you already know that switching is a layer 2 function, where frames are forwarded using just the MAC address and a dynamic table. You may also recall that routing, a layer 3 function where packets are forwarded using IP addresses, sometimes also uses some layer 4 information.

Some people will argue that there is really no such thing as layer 3 switching, and that this is all vendor-speak, just smoke and mirrors to confuse poor buyers into selecting a product. This is rather harsh, but it is true that defining layer 3 switching can be problematical.

So let’s get down to business. Why do you need layer 3 switching when you have layer 3 routing? The answer to both of these questions is simple: enhanced performance. Why do you implement any features on any piece of Cisco equipment? To improve performance and to take advantage of the robust feature set provided by Cisco. Routers, by their nature, need to analyze packets in great detail before forwarding them. This takes time, and anything that we can do to reduce the time is of benefit, especially in the modern world of QoS-hungry applications.

MLS can be implemented using more than one technology, because it really is just a vendor description for how routing can be speeded up. Cisco has two separate techniques. One involves the use of a route processor (either external or internal) that communicates specific information to a Cisco switch. The other technique is called Cisco Express Forwarding (CEF), and this requires that the switch have a routing function such as the 3550 series or the 4000 series running native IOS. In this chapter, you’ll learn about both.