MADMAN wrote: > > I seem to recall some Cisco switches that would perform > cut-through > switching until a configurable number of CRC's are detected and > would > switch to store-and-forward until errors cleared. > > Dave
Oh, that's right! I meant to mention that in the message. Some switches automatically convert to store-and-forward when a configured threshold of errors is reached. This is sometimes called adaptive cut-through switching. I don't know which switches have this features. I just know that this always comes up in theoretical discussions of the switching mode. ;-) Also, some switches offer fragment-free cut-through switching. These switches do cut-through, but only after 64 bytes have been received. That way they avoid forwarding a frame that is illegally short. The Cat 5000 and 6000 family of switches only offer store-and-forward, by the way. I think this is an argument for considering cut-through and its varieties a marketing development, rather than a technical development. The reduced latency that cut-through offers is not a big advantages on real-world networks, especially since the latency on high-end store-and-forward switches is minimal anyway. Priscilla > > Priscilla Oppenheimer wrote: > > > > Alejandro Acosta Alamo wrote: > > > > > > Hello, > > > I understand the differences between Cut-through and > Store & > > > Forward. My > > > question is: How do you decide with method to use?, in whch > > > situation have > > > you change the switching method?. > > > > > > Thanks > > > > > > Alejandro Acosta > > > > > > > > A lot of switches support only one method, so you don't have > a choice. If > > you do have a choice, the decision is based on the number of > errors on your > > network. Cut-through doesn't do any error checking and in > fact forwards > > frames that have a bad CRC or are too short. Ethernet says > that frames must > > be at least 64 bytes. Anything less is considered a fragment > and is illegal. > > Cut-through forwards fragments that have an entire > destination address that > > can be looked up to get a port number. > > > > If your switch connects many shared networks, then CRC errors > and fragments > > due to collisions are normal. But why waste bandwidth > forwarding these to > > other ports on the LAN? In this case, you might want to go > with > > store-and-forward which does not forward errored frames or > fragments. > > > > If your switch connects single devices all using full-duplex, > then it's > > unlikely that you are experiencing many CRC or fragments. So, > cut-through > > makes the most sense. > > > > Priscilla Oppenheimer > > http://www.priscilla.com > -- > David Madland > Sr. Network Engineer > CCIE# 2016 > Qwest Communications Int. Inc. > [EMAIL PROTECTED] > 612-664-3367 > > "Emotion should reflect reason not guide it" > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=48346&t=48316 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
