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.
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.”