Chapter 6. Understanding Multilayer Switching

This chapter covers the following topics:

  • Why MLS

  • Introducing MLS

  • Understanding the MLS packet flow

  • Configuring MLS

  • Router access lists

  • MLS on Catalyst 6500

  • Understanding the need for Cisco Express Forwarding

Multilayer switching (MLS) is a cache-based forwarding model. After the initial packet is handled by the router, all subsequent packets are forwarded by the hardware switching engine. The phrase "route once, switch many" comes to mind. Traditionally, the router has two primary tasks:

  • It must participate with other routers to learn and populate routes in the routing table.

  • It must software switch packets between its interfaces.

The chapter focuses on the switching component of the router and the role of a switch in taking over most of this responsibility. Newer hardware gear, such as Catalyst 6500 with Supervisor II and Supervisor 720, allows for all packets to bypass the router and directly switch in hardware.