Layer 3 Switching Key Point

A Layer 3 switch is really a router with some of the functions implemented in hardware to improve performance. In other words, some of the OSI model network layer routing functions are performed in high-performance ASICs rather than in software.

In Appendix B and Chapter 3, "IPv4 Routing Design," we describe the following various functions and characteristics of routers:

• Learning routes and keeping the best path to each destination in a routing table.

• Determining the best path that each packet should take to get to its destination, by comparing the destination address to the routing table.

• Sending the packet out of the appropriate interface, along the best path. This is also called switching the packet, because the packet is encapsulated in a new frame, with the appropriate framing header information, including MAC addresses.

• Communicating with other routers to exchange routing information.

• Allowing devices on different LANs to communicate with each other and with distant devices.

• Blocking broadcasts. By default, a router does not forward broadcasts, thereby helping to control the amount of traffic on the network.

These tasks can be CPU intensive. Offloading the switching of the packet to hardware can result in a significant increase in performance.

A Layer 3 switch performs all the previously mentioned router functions; the differences are in the physical implementation of the device rather than in the functions it performs. Thus, functionally, the terms router and Layer 3 switch are synonymous.

Layer 4 switching is an extension of Layer 3 switching that includes examination of the contents of the Layer 3 packet. For example, as described in Appendix B, the protocol number in the IP packet header indicates which transport layer protocol (for example, Transmission Control Protocol [TCP] or User Datagram Protocol [UDP]) is being used, and the port number in the TCP or UDP segment indicates the application being used. Switching based on the protocol and port numbers can ensure, for example, that certain types of traffic get higher priority on the network or take a specific path.

Depending on the switch, Layer 3 switching can be implemented in two different ways within Cisco switchesthrough multilayer switching and Cisco Express Forwarding. These terms are described in the section "Multilayer Switching and Cisco Express Forwarding." later in this chapter (after we discuss VLANs, which you must understand before you read that section).

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