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| 1.2 |  | Key Characteristics of Various Switching Technologies |
| 1.2.4 |  | Layer 4 switching |

Layer 4 switching refers to Layer 3 hardware-based routing that accounts for Layer 4 control information. Information in packet headers typically includes Layer 3 addressing, the Layer 3 protocol type, and more fields relevant to Layer 3 devices, such as Time To Live (TTL) and checksum. The packet also contains information relevant to the higher layers within the communicating hosts, such as the protocol type and port number.

The term Layer 4 switching is sometimes used to describe switching frames based on flows characterized by a combination of source/destination MAC addresses, IP addresses, and port numbers. One advantage of Layer 4 switching is it allows QoS to be applied to traffic. QoS is typically applied to classes of traffic defined by port numbers so it can be said to provide application-based prioritization.

Routers are capable of controlling traffic based on Layer 4 information. One method of controlling Layer 4 traffic is by using extended access lists. There is another method of providing Layer 4 accounting of flows, called NetFlow Switching, utilized on the Cisco 7200 and 7500 router platforms.

Finally, when performing Layer 4 functions, a switch reads the TCP and UDP fields within the headers to determine what type of information the packet is carrying. The network manager can program the switch to prioritize traffic by application. This function allows network managers to define a QoS for end users. When used for QoS purposes, Layer 4 switching might mean that a videoconferencing application is granted more bandwidth than an e-mail message or File Transfer Protocol (FTP) packet.

Layer 4 switching is necessary if the policy of the organization dictates granular control of traffic by application or if the organization requires accounting of traffic itemized in terms of applications. However, it should be noted that switches performing Layer 4 switching need the ability to identify and store large numbers of forwarding-table entries, especially if the switch is within the core of an enterprise network. Many Layer 2 and Layer 3 switches have forwarding tables that are sized in proportion to the number of network devices.

With Layer 4 switches, the number of network devices must be multiplied by the number of different application protocols and conversations in use in the network. Thus, the size of the forwarding table can grow quickly as the numbers of end devices and types of applications increase. This large table capacity is essential to creating a high-performance switch that supports wire-speed Layer 4 forwarding of traffic.