Understanding the network.pdf - Back to Home

switch configuration, add the configured port speeds of the switch. If
the number exceeds your switch's rated capacity, reconfigure the
switch to avoid blocking.
Ideally, a switch should have enough backplane capacity to support
full-duplex operation between all its ports. This is generally the case
with most workgroup and enterprise-class switches on the market
today. When LAN switches first came to market, they did not have
adequate backplane (or processing capability) support to provide wire
rate connections between all the switches ports simultaneously. This
is also the case with many of today's inexpensive desktop-class
switches. To accommodate data transmission in these cases, port
blocking is employed when the switch's backplane becomes
overloaded, to minimize frame loss. This problem also exists with
many Gigabit Ethernet switch and uplink port implementations. A
single Gigabit connection can transfer a maximum of 1,481,000 FPS,
which is more than the minimum required backplane capacity for
half-duplex operation of a 24-port Fast Ethernet switch.
Implementations are getting faster, however, and with time wire rate
Gigabit Ethernet will be available.
Caution should be taken when using vendor performance calculations
to determine how to provision the ports on your switch. Rarely do
switches perform at the rates their manufacturers claim. To be safe,
use an adjusted backplane throughput rate. Assume that the switch
will perform at 80 percent of its stated rate and then configure port
speeds so utilization will not exceed this number.
Switch Forwarding Methodologies
Whereas the backplane capacity determines the switch's performance
in terms of how much data will fit through the switch, the forwarding
methodology dictates how long it takes to process each packet (the
latency). It is the combination of these two elements that determines
the switch's FPS rate. There are three types of frame forwarding
methods employed by switch designers: cut-through,
store-and-forward, and hybrid or modified cut-through. Most
manufacturers of workgroup and enterprise LAN switches support all
three forwarding methods either through manual configuration or
automatically, based on traffic flow and traffic quality. However, there
is a core of manufacturers that only build store-and-forward switches
because this functionality is required for their proprietary
enhancements.