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 -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: http://lists.debian.org/4c847f8a.3060...@hardwarefreak.com
