Content

| Overview

The desire for high-speed networking provides the movement to develop new technologies. One of the bottlenecks in high-speed networking is the decision-making process within the router. Until recently, routers forwarded traffic without the aid of specialized hardware designed to rewrite Protocol Data Unit (PDU) headers to speed up the process. In recent years, technologies and accompanying hardware have enhanced the process of rewriting PDU headers to move traffic to its ultimate destination.

A number of software and hardware combinations enable hardware-based PDU header rewrites and forwarding. Two of the methods used by Cisco devices are Multilayer Switching (MLS) and Cisco Express Forwarding (CEF). The hardware requirements for each of these technologies are strict, so it is important to do some research before making a purchase. This module clarifies the hardware and software requirements for supporting MLS and CEF.

MLS is a technology used by a small number of older Catalyst switches to provide wire-speed routing. MLS is sometimes known as "Route once, switch many", where the first packet of a flow is routed by the router in software and the remaining packets are forwarded in hardware by the switch.

CEF is the technology used by newer Cisco devices to provide wire-speed routing. MLS looks at the first packet in a flow and caches the requisite information to permit subsequent packets to be switched independent of the route processor. CEF enables packet switching to circumvent the route processor altogether. This is accomplished by using specialized data structures that are dynamically updated by a communication process between the route processor and the switch processor. Therefore, the traditional routing of the “first” packet required by MLS is avoided. In general, CEF can be thought of as "Route never, switch always", where the packet is never routed by the router in software, and always forwarded in hardware by the switch.

This module shows how MLS and CEF work and how to configure these technologies. Verification, troubleshooting, and interpreting the output of various show commands relating to MLS and CEF will also be covered.