Layer 4 Switching

Layer 4 Switching
Layer 4 switching is considered a hardware-based layer 3 switching technology that can also
consider the application used (for example, Telnet or FTP). Layer 4 switching provides additional
routing above layer 3 by using the port numbers found in the Transport layer header to
make routing decisions. These port numbers are found in Request for Comments (RFC) 1700
and reference the upper layer protocol, program, or application.
Layer 4 information has been used to help make routing decisions for quite a while. For example,
extended access lists can filter packets based on layer 4 port numbers. Another example is accounting
information gathered by NetFlow switching in Cisco’s higher end routers.
The largest benefit of layer 4 switching is that the network administrator can configure a
layer 4 switch to prioritize data traffic by application, which means a QoS can be defined for
each user. For example, a number of users can be defined as a Video group and be assigned more
priority, or bandwidth, based on the need for videoconferencing.
However, because users can be part of many groups and run many applications, the layer 4
switches must be able to provide a huge filter table or response time would suffer. This filter
table must be much larger than any layer 2 or 3 switch. A layer 2 switch might have a filter table
only as large as the number of users connected to the network, maybe even smaller if some hubs
are used within the switched fabric. However, a layer 4 switch might have five or six entries for
each and every device connected to the network! If the layer 4 switch does not have a filter table
that includes all the information, the switch will not be able to produce wire-speed results.