Cut-Through or Store-and-Forward: Ethernet Switching for Low-Latency

This section provides an overview of the functions and features of store-and-forward Ethernet switches.

Figure 1 shows a store-and-forward switch receiving an Ethernet frame in its entirety. At the end of that frame, the switch 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 switch then performs the forwarding process.

Whereas a store-and-forward switch drops invalid packets, cut-through devices forward them because they do not get a chance to evaluate the FCS before transmitting the packet.

The process of storing and then forwarding allows the switch to handle a number of networking conditions simply by the way it operates.

The ingress buffering process that a store-and-forward switch performs provides the flexibility to support any mix of Ethernet speeds, starting with 10 Mbps. For example, handling an incoming frame to a 1-Gbps Ethernet port that needs to be sent out a 10-Gbps interface is a fairly straightforward process. The forwarding process is made easier by the fact that the switch's architecture stores the entire packet.

Because a store-and-forward switch stores the entire packet in a buffer², it does not have to execute additional ASIC or FPGA code to evaluate the packet against an access control list (ACL). The packet is already there, so the switch can examine the pertinent portions to permit or deny that frame.