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| 5.1 |  | Multilayer Switching |
| 5.1.3 |  | MLS components |

The idea of MLS is simple. MLS looks at the first packet in a flow of data and caches some header information describing the flow. Subsequent packets in the flow circumvent the router because the switch has cached the data necessary to rewrite the packet header, which it does. This is known as wire-speed switching.

MLS switches unicast IP data packet flows between IP subnets using advanced ASIC switching hardware, offloading processor-intensive packet routing from network routers. The packet forwarding function is moved onto Layer 3 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 are still forwarded in software by routers. Standard routing protocols, such as OSPF, EIGRP, RIP, and IS-IS are used for route determination.

IP is a connectionless protocol and every packet is delivered independent of every other packet. However, actual network traffic consists of many end-to-end conversations, or flows, between users or applications.

A flow is a unidirectional sequence of packets between a particular source and destination that share the same Layer 3 and Layer 4 information. For example, Telnet traffic transferred from a particular source to a particular destination is a separate flow from FTP packets between the same source and destination. Communication from a client to a server, and from the server to the client, comprise separate and distinct flows.

If a flow is defined administratively by destination IP address alone, traffic from multiple users or applications to that destination is included in that flow.

The MLS Switching Engine (MLS-SE) is the set of hardware components on a Catalyst switch, excluding the route processor, necessary to support MLS. For example, a Catalyst 5000 with a Supervisor Engine IIIG is an MLS-SE. These requirements were detailed in the preceding section. The MLS Route Processor (MLS-RP) is a Cisco device with a route processor that supports MLS. For example, a Cisco 3620 router is an MLS-RP. The devices meeting these criteria are also detailed in the preceding section. Only certain combinations of MLS-SEs and MLS-RPs can run MLS.