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Store-and-forward switching

Data Frames Forwarding and Switching Method

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.

Switching Fundamentals – Everything to Know

Switching Fundamentals – Everything to Know

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An Ethernet switch is a Layer 2 device. Therefore, the switch uses MAC addresses for switching decisions. It is unaware of the protocol being carried in the data portion of the frame, such as an IPv4 packet. The Ethernet hub repeats bits out of all ports except the incoming port, but an Ethernet switch consults a MAC address table to make a forwarding decision for each frame. The MAC address table is sometimes called a content addressable memory (CAM) table.

Learning the Source and destination MAC Addresses

The switch automatically builds a MAC address table by examining the source MAC address of the frames received on any port. It forwards frames by searching for a match between the destination MAC address in the frame and an entry in the MAC address table.  It checks each entering frame for new information, for example, examining the frame’s source MAC address and port number where the frame entered the switch.

If the source MAC address does not exist in the MAC address table, it and the incoming port number are added to the table. If the MAC address is found in the table, the frame will be sent out of the specified port.

If the source MAC address does exist, the switch updates the refresh timer for that entry. By default, most Ethernet switches keep an entry in the table for 5 minutes. If the source MAC address exists in the table but on a different port, the switch treats this as a new entry. The entry was replaced using the same MAC address but with the more current port number.

Frame Forwarding Methods on Cisco Switches

Cisco switches support different frame-forwarding Methods. Frame Forwarding Methods determine how a switch receives, processes, and forwards a Layer 2 Ethernet frame. Important Switching methods are the following:-

Store-and-forward switching

The switch copies each incoming Ethernet frame into the switch memory in this method. During storage, the switch analyzes the frame for information about its destination and computes the Cyclic Redundancy Check (CRC) trailer for errors. If a Cyclic Redundancy Check (CRC) error is found, the Ethernet frames are then dropped, and if there is no Cyclic Redundancy Check (CRC) error, the switch forwards the Ethernet frame to the destination device.

The store-and-forward method can cause a delay because Cyclic Redundancy Check (CRC) takes time to calculate each Ethernet frame and is also the most processor-intensive. CRC uses a mathematical formula based on the number of bits (1s) in the frame to decide whether the received frame has an error.

After confirming the reliability of the frame, the frame was forwarded to the correct port and toward its destination. Discarding frames with errors reduces the amount of bandwidth consumed by corrupt data. The store-and-forward method requires Quality of Service (QoS)

Cut-Through Switching

In cut-through, the frame header is inspected, and the frame’s Destination MAC Address is copied into the switch’s internal memory before the frame is forwarded. This is the fastest switching method because the switch only analyzes the destination MAC address located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its Mac address table, determines the outgoing interface port, and forwards the frame to its destination through the designated switch port.

However, with speed comes some significance: the switch also forwards frames with errors. Because the switch does not perform any error checking on the frame, it is up to the destination switch to discard received frames with errors.

Switch operating in cut-through mode reduces delay because the switch starts to forward the Ethernet frame as soon as it reads the destination MAC address. The problem with the cut-through method is that the switch may forward bad frames. The cut-through method is the predominant switching method used on Cisco switches. There are two variants of cut-through switching:

Fast-forward switching

Fast-forward gives the lowest latency in switching because it starts forwarding before the entire packet has been received. There may be times when packets are relayed with errors, but this occurs infrequently, and the destination network adapter discards the faulty packet upon receipt. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted. Fast-forward is the typical cut-through method of switching.

Fragment-free switching

Fragment-free is an advanced form of cut-through switching. It waits for the collision window, the first 64 bytes of a frame, to be accepted before forwarding the frame to its destination. The fragment-free method holds the packet in memory until the data portion reaches the switch. It only reads the destination MAC address field in the Ethernet frame before making a switching decision.

The switches operating in fragment-free mode read and store at least 64 bytes of the Ethernet frame before switching it to avoid forwarding Ethernet “runt” frames, which are smaller than 64 bytes.

Fragment-free switching can be viewed as a compromise between store-and-forward and fast-forward methods. Fragment-free switching stores only the first 64 bytes of the frame because most network errors and collisions occur during these bytes.

Fragment-free switching tries to improve fast-forward switching by performing a small error check on the first 64 bytes of the frame.  Fragment-free switching compromises the high latency and high integrity of store-and-forward switching and the low latency and reduced integrity of fast-forward switching.

Some switches require configuration to do cut-through switching on a per-port basis until a user-defined error threshold is reached; then, they automatically change to store-and-forward. When the error rate falls below the threshold, the port returns to cut-through switching.