Data Frames Forwarding and Switching Method

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With the growth of the network, its performance is slower, and Ethernet bridges limit the size of the collision domains. Therefore, the use of ethernet bridges art started. The advancement in integrated circuits permitted LAN switches to replace the early bridges. The modern switches moved the layer 2 forwarding decision from software to application-specific-integrated circuits (ASICs). The ASICs decrease the packet-handling time within the device and let the device handle an increased number of ports without degrading performance.  There are two methods of frame forwarding and switching:-

  • Store-and-forward method
  • Cut-through Method

Store-and-Forward Switching

The store-and-forward method makes a forwarding decision when the complete frame is received. Then, it checks the frame for errors using a mathematical error-checking mechanism known as a cyclic redundancy check (CRC). If the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. The frame is then forwarded to the correct port.

The Store-and-forward method has two primary characteristics that differentiate it from cut-through:

  • Error checking
  • Automatic buffering.

Error Checking

A switch using a store-and-forward switching technique performs an error check on each incoming frame. When receiving the entire frame on the ingress port, it shows the figure; the switch compares the frame-check-sequence (FCS) value in the last field of the datagram against its own FCS calculations. The FCS is the process that helps ensure the frame is free of physical and data-link layer errors. If the frame has no error, the switch forwards the frame to the destination; otherwise, the frame is dropped.

An illustration of the Store and Forward Switching method showing a layered diagram with four levels labeled from top to bottom Data Transport Header Network Header and Frame Header Each header layer contains different components Data is standalone Transport Header includes Type Network Header includes Source MAC Address and Destination MAC Address Frame Header includes Preamble FCS Checksum CRC The image highlights the process where the store and forward method makes a forwarding decision when the complete frame is received
Store and Forward Switching Method Diagram

Automatic Buffering

If the ingress and egress ports have different data speeds, the switch stores the frame in a buffer computes the FCS check, forwards it to the egress port buffer, and then sends it. For example, when an incoming frame traveling into a Fast Ethernet port must be sent out, a Giga Ethernet interface must use the store-and-forward method. Store-and-forward switching is the primary method for Cisco switches.

Cut-Through Switching

Cut-through switching is another switching method, as shown in Figure 2; this method starts the forwarding process when the destination MAC address of an incoming frame and the egress port have been determined. The advantage of this method is the ability to switch data earlier than the store-and-forward method. The primary characteristics of cut-through switching are the following:

  • Rapid Frame Switching
  • Fragment Free
A diagram illustrating the process of Cut through Switching in network communication The image shows a pyramid like structure with layers labeled from top to bottom Frame Header Network Header Transport Header and Data Each header is further divided into components such as Preamble Destination MAC Address Source MAC Address Type and FCS Checksum CRC The bottom of the image includes a note stating On Receiving of destination MAC address switch start forwarding frame
Cut through Switching Process Diagram

Rapid Frame Forwarding

A cut-through switch moves forward immediately when it finds the frame’s destination MAC address in its MAC address table. Unlike the store-and-forward method, the switch doesn’t need to wait for the complete frame.

Because of ASICs and MAC controllers, a switch using the cut-through method can quickly be decided. The cut-through method needs to check a larger part of a frame’s headers for more filtering purposes. For example, the switch can check the source MAC address, destination MAC, and the Ether Type fields, which total 14 bytes, and check an extra 40 bytes to carry out more difficult functions in Layers 3 and 4.

This method does not drop invalid frames. The frames with errors are forwarded to the next segments of the network. If there are too many invalid frames in the network, this hurts bandwidth.

Fragment Free Switching

It is a modified form of cut-through switching. In this switching form, the switch waits for the collision window (64 bytes) to pass before forwarding the frame. Each frame will be checked into the data field to make sure no fragmentation has occurred. This provides enhanced error checking rather than cut-through without further latency and delay. The lower latency speed of cut-through switching makes it more suitable for high-performance computing (HPC) applications that need process-to-process latencies of 10 microseconds or less.

FAQs – Data Frames Forwarding and Switching Method

Q1: What is data frame forwarding? 

A1: Data frame forwarding is moving data packets from one network segment to another based on the destination address contained within the frame. Network switches and routers typically handle this.

Q2: What are the main methods of data frame forwarding and switching? 

A2: The main methods are Store-and-Forward Switching and Cut-Through Switching. Store-and-Forward involves checking the entire frame for errors before forwarding, while Cut-Through starts forwarding as soon as the destination address is read.

Q3: How does Store-and-Forward Switching work? 

A3: Store-and-Forward Switching works by receiving the entire data frame, checking it for errors using a cyclic redundancy check (CRC), and then forwarding it if no errors are found. This method ensures data integrity but can introduce latency.

Q4: What is Cut-Through Switching? 

A4: Cut-through switching begins forwarding the frame as soon as the destination MAC address is read without waiting for the entire frame to be received. This reduces latency but does not check for errors in the frame.

Q5: What are the advantages of Store-and-Forward Switching? 

A5: The advantages include error checking and automatic buffering, which help ensure data integrity and manage differences in data transmission speeds between network segments.

Q6: What are the advantages of Cut-Through Switching? 

A6: The primary advantage is reduced latency, as frames are forwarded almost immediately after the destination address is read. This can be beneficial in high-speed networks where speed is required.

Q7: What is the difference between Rapid Frame Forwarding and Fragment Free Switching? 

A7: Rapid Frame Forwarding is a characteristic of Cut-Through Switching, where frames are forwarded as soon as the destination address is read. Fragment-free Switching is a variation in which the switch waits until the first 64 bytes of the frame are received to ensure no collision fragments are present.

Q8: When should I use Store-and-Forward Switching over Cut-Through Switching? 

A8: Store-and-forward switching is preferable when data integrity is crucial, such as in networks where error checking is important. Cut-through switching is better suited for environments where low latency is critical than error checking.

Q9: Can modern switches use both Store-and-Forward and Cut-Through Switching? 

A9: Many modern switches can use both methods and switch between them based on the network requirements and conditions.

Q10: What role do ASICs play in data frame forwarding? 

A10: Application-specific integrated Circuits (ASICs) are used in modern switches to handle the forwarding decision process at hardware speed, significantly reducing packet-handling time and allowing the switch to manage more ports efficiently.