When Layer 2 Ethernet switch initiates the forwarding decision, The Ethernet switch undergoes series of steps to determine whether to forward or drop a packet is what differentiates the cut-through Switch from store-and-forward Switch.
Both store-and-forward and cut-through Layer 2 switches takes their forwarding decisions on the destination MAC address of data packets. They also learn MAC addresses as they examine the source MAC (SMAC) fields of packets as stations communicate with other nodes on the network.
In theory Cut-through switch receives and examines only the first 6 bytes of a frame, which carries the DMAC (Destination MAC ) address. However, practically cut-through switches wait until a few more bytes of the frame have been evaluated before they decide whether to forward or drop the packet.
Enhanced Cut-through (Fragment Free) Switches:
Enhanced cut-through switches checks the frame up to 64 bytes and looking for any errors. if not found any errors then starts forwarding it to the destination. Enhanced cut-through Switches having little more delay than cut-through switches but some about reliable.
Store-and-forward switch makes a forwarding decision on a data packet after it has received the whole frame, stores in buffer and checked its integrity, a cut-through switch engages in the forwarding process soon after it has examined the destination MAC (DMAC) address of an incoming frame.
Difference Between Cut-through and Store-and-forward Switches:
Store-and-Forward Switch receives entire frame and it will compare the last field of the datagram against its own frame-check-sequence (FCS) calculations, to help ensure that the packet is free of physical and data-link errors. The Store-and-Forward switch then performs the forwarding process and it drops invalid packets.
Cut-through Switch devices forward them because they do not get a chance to evaluate the FCS before transmitting the packet. At the receiving end, the host invalidates the FCS of the packet and drops the packet.
Packet Forwarding Time:
A cut-through switch can make a forwarding decision as soon as it receives the DMAC address of the data packet. The switch does not have to wait for the rest of the packet to make its forwarding decision. Cut-through switches takes very low latency when compared to Store-and-forward Switches, it is few microseconds only regardless of the packet size.
Filed Under: LAN General
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