cut-through switching

[NewBoy]

Hi,

Networking novice here. Been reading up on Cisco switching (though I guess this is a generic switching question).

As cut-through switching achieves low latency by only reading as far as the dest address, how does such a switch form a table of source addresses? If it doesn't read the source address, surely it can never build up a table matching ports to mac addresses.

I'm sure it's not that simple, but I can't figure out what I'm missing here! All useful (i.e. non-RTFM) comments welcomed.

Thanks.

 

[wybnormal]

The short version is that cut-through reads the header and switches the packet *before* the rest of the data payload is read into the switch. Header info is header info and siwtch maintains the normal list of of macs and ports. Contrast this to store and forward where the header AND data is read completely, CRC checked and then stored until resources are available to switch the packet. SF will have a longer latency then cut-through.. Cut-through will have a constand latency since the header is almost always the same length each time so it's always the same amount of time to process the header info.

For some more details:

http://www.cicprep.com/WebHelp/ccna/ccna_obj_57.htm

Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin

 

[NewBoy]

Right,

Thanks wybnormal - didn't know the rest of the frame got read!