CCNA Certification Exam Tutorial Cisco Switching Modes
You must be fluent in switching to pass the CCNA exam. You should spend time learning the basics of switching theory and the three possible switching modes Cisco routers offer.
It is exactly what Store-and-Forward sounds like. Before it can be forwarded, the entire frame will be saved. This mode provides the most error-checking, as a Cyclical Redundancy Test (CRC) is run on the frame before it’s forwarded. The frame is discarded if it contains an error. If the frame has no problems, it is forwarded to the correct destination.
Although store-and-forward performs error checking, the latency caused by the delay in processing frames while the error check is running results in a higher latency than other modes. Because not all frames have the same size, latency times can vary.
Cut-through switching copies the destination MAC address only into its memory before it begins to forward the frame. There is less latency with store-and-forward because the frame is forwarded immediately after the destination MAC address is read. There is no error checking.
There is a middle ground: fragment-free switching. It copies only a portion of the frame to memory before it can be forwarded. However, it’s the first 64 bytes and not the destination MAC. (Why? Why? Although there is more error checking than with cut-through, there is not as much latency when compared to store-and-forward.
Both cut-through (and fragment-free) have a fixed latency. These modes only look at the first 6 or 64 bytes. The frame size determines the latency of Store-and-forward.
It is a common mistake to overlook the differences and similarities between these modes when studying CCNA. This important topic is a key part of CCNA studies. You’ll be happy you did.