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Multilayer Switching Overview


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Multilayer Switching Overview






Key MLS Features



Introduction to Multilayer Switching



Multilayer Switching Implementation



Standard and Extended Access Lists



Restrictions on Using IP Router Commands with MLS Enabled



General Guidelines



Software and Hardware Requirements



Guidelines for External Routers



Features That Affect MLS

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Cisco IOS 12.0 Switching Services

From: Cisco IOS 12.0 Switching Services
Author: Technologies Riva; Systems Cisco
Publisher: Cisco Press (53)
More Information

12. Multilayer Switching Overview

This chapter provides an overview of Multilayer Switching (MLS).


The commands and configurations described in this chapter apply only to the devices that provide routing services. Commands and configurations for Catalyst 5000 series switches are not documented here.

MLS provides high-performance Layer 3 switching for the Catalyst 5000 series LAN switches. MLS switches IP data packets between subnets using advanced application-specific integrated circuit (ASIC) switching hardware. Standard routing protocols, such as Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), Routing Information Protocol (RIP), and Intermediate System-to-Intermediate System (IS-IS), are used for route determination.

MLS enables hardware-based Layer 3 switching to offload routers from forwarding unicast IP data packets over shared-media networking technologies such as Ethernet. The packet forwarding function is moved onto Layer 3 Catalyst 5000 series switches whenever a partial or complete switched path exists between two hosts. Packets that do not have a partial or complete switched path to reach their destinations still use routers for forwarding packets.

MLS also provides traffic statistics as part of its switching function. These statistics are used for identifying traffic characteristics for administration, planning, and troubleshooting. MLS uses NetFlow Data Export (NDE) to export the flow statistics.

The Route Switch Module (RSM) performs route processing and central configuration and control for the Catalyst 5000 series switch. Routing services can also be provided by an externally attached router.

MLS consists of the following:

  • Catalyst 5000 series multilayer LAN switches

  • Catalyst RSM, which provides Cisco IOS-based multiprotocol routing and network services


Cisco 7500, 7200, 4500, and 4700 series routers also support MLS.

  • NetFlow Feature Card (NFFC), which is a modular feature-card upgrade for the Catalyst Supervisor Engine III to provide Layer 3 switching


The 10/100BaseTX and 100BaseFX Backbone Fast Ethernet Switching modules have onboard hardware that optimizes MLS performance.

Procedures for configuring MLS and NDE on routers are provided in Chapter 13, “Configuring Multilayer Switching.”


The following terminology is used:

  • Multilayer Switching-Switching Engine (MLS-SE)—An NFFC-equipped Catalyst 5000 series switch.

  • Multilayer Switching-Route Processor (MLS-RP)—A Cisco router with MLS enabled.

  • Multilayer Switching Protocol (MLSP)—The protocol running between the MLS-SE and MLS-RP to enable MLS


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