| 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.
A simple definition of Layer 4
switching is the ability to make forwarding decisions based not just
on the MAC address or source/destination IP addresses, but on Layer
4 parameters such as port numbers as well. In TCP or User Datagram
Protocol (UDP) flows, the application is encoded as a port number in
the segment header.
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
your policy dictates granular control of traffic by application or
if you require 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.
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