Abhi-2809
commented
2 years ago Hello, Hope you are doing well, I wanted to enquire if you have an python implementation of Ethernet switch forwarding. It would be extremely helpful if you could share the code or any resources that could help us implement the code. Eagerly looking forward to hearing from you
Best, Abhinav
Layer 2 Forwarding
Both store-and-forward and cut-through Layer 2 switches base 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.
When a Layer 2 Ethernet switch initiates the forwarding decision, the series of steps that a switch undergoes to determine whether to forward or drop a packet is what differentiates the cut-through methodology from its store-and-forward counterpart.
Whereas a store-and-forward switch makes a forwarding decision on a data packet after it has received the whole frame 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.
In theory, a cut-through switch receives and examines only the first 6 bytes of a frame, which carries the DMAC address. However, for a number of reasons, as will be shown in this document; 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.
Characteristics of Store-and-Forward Ethernet Switching
This section provides an overview of the functions and features of store-and-forward Ethernet switches.
Error Checking
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.
Figure 1. Ethernet Frame Entering a Store-and-Forward Bridge or Switch (from Left to Right)
Automatic Buffering
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.
Access Control Lists
Because a store-and-forward switch stores the entire packet in a buffer 2, 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.
Characteristics of Cut-Through Ethernet Switching
This section explores cut-through Ethernet switching. Because cut-through switching is not as well understood as store-and-forward switching, it is described in more detail than the store-and-forward technology.
Invalid Packets
Unlike store-and-forward switching, cut-through switching flags but does not get a chance to drop invalid packets. Packets with physical- or data-link-layer errors will get forwarded to other segments of the network. Then, at the receiving end, the host invalidates the FCS of the packet and drops the packet.
Timing of Cut-Through Forwarding
In theory, as indicated in Figure 2, a cut-through switch can make a forwarding decision as soon as it has looked up 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.
However, newer cut-through switches do not necessarily take this approach. A cut-through switch may parse an incoming packet until it has collected enough information from the frame content. It can then make a more sophisticated forwarding decision, matching the richness of packet-handling features that store-and-forward switches have offered over the past 15 years.
Figure 2. Cut-Through Ethernet Switching: in theory, frames are forwarded as soon as the switch receives the DMAC address, but in reality, several more bytes arrive before forwarding commences