Understanding the network.pdf - Back to Home

• Switch backplane—Also referred to as a switch matrix. This is
the interconnect used by the switch ports to establish
connection paths and exchange packets.
• Switching logic—Contains four elements:
o A CPU manages the operation of the switch and packet
forwarding.
o The switch logic module is generally an
Application-Specific Integrated Circuit (ASIC) that
performs Layer 2 address lookups, SAT table creation,
and sets up and tears down port connections across the
backplane as needed for packet forwarding. ASIC-based
switches are the predominant type of switch on the
market, mainly because they are less expensive and
perform better than using RISC or CISC processors and
software. Because the use of ASIC technology enables the
switch, you can perform all the switch operations in
hardware. The downside is that the switch vendor needs
to update or re-spin the ASIC every time a new
technology arrives that cannot be handled with software.
This can become an annoyance because often you need to
buy a new switch for any significant technology or feature
change.
o DRAM is used for packet buffer and SAT storage. Two
different approaches are used to implement port
buffering: dedicated and shared. With dedicated buffering,
each port has its own dedicated DRAM buffer. With shared
buffering, all the ports use a common memory pool.
Dedicated buffering is faster, but is prone to overflows
(which essentially halts packet processing) because the
buffers are often not very large (32KB is the average size).
The shared approach is slower but more efficient because
the pool is shared to allow the ports that need the buffer
to get it. With the dedicated model, the buffer goes
unused. Packet buffer size is very important to the
efficient operation of the switch the more the better.
Inadequate port buffering results in packet loss,
excessive collisions, and overall poor switch performance.
o Flash memory is used to hold the switch's operating
system and switch configuration information. Older
switches can use EPROMs to hold the switch's OS and
NVRAM to hold configuration information. This, however,
is quite a drag when the time comes for an OS upgrade
because the PROM usually needs to be replaced.