Cisco Switch Frame Forwarding

We have different Frame Forwarding Option, another words When Frame comes in one of those Ethernet switches port. Switch begin to forward the frame out of the egress or the outgoing port.There is Different approach to do this

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Cut-Through Switching: -   

   Does not use FCS (frame Check Sequence)

   Start to forward the frame after examine the frame’s Destination mac address 

                   Basic Cut-Through Switching what we responsible for understanding CCNA exam is that with Cut-Through switching, as soon as switch sees those first 6 Bytes (48 bits), as soon as sees destination mac address, it can start forwarding that frame.

                  Therefore, upside is decreasing latency and the downside is we might forward the frame that’s corrupted frame

                   Ether Type: - This is an example, IPv4 Frame, may be ACL for blocking this frame, may be a Qos.

Store-and-Forward Switching

                     With Store and forward switching switch once to make sure that a frame is valid and that’s not corrupt, before it starts to forwarded it out. Switch wait until it receives the entire frame not just the first 48 bits or destination mac address, it waits until receives the entire frame and looks at the frame Check sequence embedded in frame, and switch is going to calculate using an algorithm, its gonna calculate its own Frame Check Sequence (FCS). So the Switch creates FCS of its own, and if the FCS in the frame equals the FCS calculates with the switch. 

                     The switch can conclude with reasonable certainty that this is a valid frame it is not being corrupted in transmission. Therefore I feel comfortable forwarding an out for the destination.Store and Forward Switching upside is that we were not going to waste bandwidth by beginning forward a frame that’s invalid anyway.

                        The Downside is that it might be little bit slower than Cut-Through Switching because we are waiting until we receiving the entire frame before we begin forwarding that frame.

Fragment-Free Switching  

                       Starts to forward the frame after the first 64 Byte are received, since most collision occur within the 64 Bytes                            

                      We could almost consider Fragment Free Switching as a compromise between Cut-Through Switching and Store and Forward Switching. One of big benefits of Cut-Through Switching was that switch didn’t have to see the entire frame before it started to forward it out towards its destination. However, it didn’t check to see if it valid or not, maybe collision had occurred may be FCS didn’t match up. With Store and Forward switching we would get the entire frame stored in the switch make sure it good then start forwarding it but it introducing some delay by having to receive the entire frame before we start to forward the frame.

                   With Fragment-Free Switching stats that most collisions that going to occurred within a frame, are going to occurred within first 64 Byte. Fragment switching does, it looks beyond the just destination mac address, and beyond the ether type, it will actually look 64 Bytes into our frame, because most of the collisions occurred in the first 64 Bytes, if it doesn’t see the collision has occurred in first 64 bytes, it has some assurance that this is probably an OK frame. We didn’t go to trouble for wait for FCS arrive and do calculation, as we would with in Store and Forward Switching. But at least we have some assurance that frame is valid as supposed to Cut-Through Switching

                     To Some Up Fragment Free Switching is going to examine the first 64 Bytes of a frame, if it doesn’t seem that a collision has occurred, it is then going to forward that frame based on destination mac address 


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