Devices involved in Layer 4 switching perform the following functions:
• Packets are forwarded using hardware switching, based on both Layer 3 addressing and Layer 4 application information.
• Layer 3 protocol types (UDP or TCP, for example) in packet headers are examined.
• Layer 4 segment headers are examined to determine application port numbers.
Switching at Layer 4 allows finer control over the movement of types of information. For example, traffic can be prioritized according to the source and destination port numbers, and QoS can be defined for end users. Therefore, video or voice data can be switched at a higher level of service with more bandwidth availability than file transfer or HTTP traffic. Layer 4 port numbers for source and destination can also perform traffic accounting.
A Layer 4 switch must also allocate a large amount of memory to its forwarding tables. Layer 2 and Layer 3 devices have forwarding tables based on MAC and network addresses, making those tables only as large as the number of network devices. Layer 4 devices, however, must also keep track of application protocols and conversations occurring in the network. Their forwarding tables become proportional to the number of network devices multiplied by the number of applications.
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