Re: Re (2): Linux hub
Boyd Stephen Smith Jr. put forth on 9/5/2010 2:56 PM:
> Switches are smart devices, but not traditionally programmable. They do use
> some RAM to store information about seen packets in order to make decisions
> about future packets.
Ethernet switches are not "smart" devices at all, unless you're talking
about more expensive models such as Cisco units that support things such
as VLANs, layer 2 QOS, multi switch stacking, etc.
A traditional el cheapo desktop 8 port ethernet switch, such as the $10
Rosewill 10/100 switch on my desk, has a single simple 8x8 crossbar
switch ASIC, enough RAM to store 8192 MAC addresses, and possibly a
small frame buffer per port (2k to 16k), since the switching mode is
store and forward--buffering even just one 1500 byte frame can
substantially increase performance under load.
Layer 2 ethernet switches, the bulk of all sold to date, don't have any
knowledge of "packets". They don't store information about "seen
packets". The only information they store is the MAC addresses of all
the devices which have broadcast their MAC address over the wire at
power on.
Packets exist at layer 3 in OSI, and can be from any number of protocols
including IPX/SPX, TCP/IP, encapsulated fiber channel, PPPoE, PPTP, etc,
etc. Switches are ignorant of packets. Switches accept and pass
ethernet _frames_ which contain the packets. And every frame passing
through a switch does go through the "CPU" of the switch, called the
crossbar, which is a very simple ASIC in low end switches.
Larger switches, such as some 576 (max) port core switches (24 blades
w/24 ports each), have rather large central crossbar ASICs, or multiple
smaller ones connected in a grid. The switch ASIC on each blade card,
say a 24 port 10/100/1000 switch blade, will likely be a 26x26 crossbar,
with 24 full duplex 10/100/100 device ports, and two full duplex 10 Gb/s
back plane connections to the central switch ASIC(s).
The larger switch ASICs cost far more than those in $10 eight port
switches, have more memory for storing MAC addresses, and have extensive
management processing power. But the core design is the same, a
crossbar switching ethernet frames in one port and out another, with or
without buffering.
Switches aren't really smart devices. For "smart", you need something
processing layer 3 and higher protocols, such as a router or firewall.
--
Stan
Reply to: