Switch Forwarding Methods

Switch Packet Forwarding Methods
In this topic, you will learn how switches forward Ethernet frames on a network. Switches can operate in different modes that can have both positive and negative effects.
In the past, switches used one of the following forwarding methods for switching data between network ports: store-and-forward or cut-through switching. Referencing the Switch Forwarding Methods button shows these two methods. However, store-and-forward is the sole forwarding method used on current models of Cisco Catalyst switches.
Store-and-Forward Switching
In store-and-forward switching, when the switch receives the frame, it stores the data in buffers until the complete frame has been received. During the storage process, the switch analyzes the frame for information about its destination. In this process, the switch also performs an error check using the Cyclic Redundancy Check (CRC) trailer portion of the Ethernet frame.
CRC uses a mathematical formula, based on the number of bits (1s) in the frame, to determine whether the received frame has an error. After confirming the integrity of the frame, the frame is forwarded out the appropriate port toward its destination. When an error is detected in a frame, the switch discards the frame. Discarding frames with errors reduces the amount of bandwidth consumed by corrupt data. Store-and-forward switching is required for Quality of Service (QoS) analysis on converged networks where frame classification for traffic prioritization is necessary. For example, voice over IP data streams need to have priority over web-browsing traffic.
Cut-through Switching
In cut-through switching, the switch acts upon the data as soon as it is received, even if the transmission is not complete. The switch buffers just enough of the frame to read the destination MAC address so that it can determine to which port to forward the data. The destination MAC address is located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame onto its destination through the designated switch port. The switch does not perform any error checking on the frame. Because the switch does not have to wait for the entire frame to be completely buffered, and because the switch does not perform any error checking, cut-through switching is faster than store-and-forward switching. However, because the switch does not perform any error checking, it forwards corrupt frames throughout the network. The corrupt frames consume bandwidth while they are being forwarded. The destination NIC eventually discards the corrupt frames.
There are two variants of cut-through switching:
Fast-forward switching: Fast-forward switching offers the lowest level of latency. Fast-forward switching immediately forwards a packet after reading the destination address. Because fast-forward switching starts forwarding before the entire packet has been received, there may be times when packets are relayed with errors. 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 switching is the typical cut-through method of switching.
Fragment-free switching: In fragment-free switching, the switch stores the first 64 bytes of the frame before forwarding. Fragment-free switching can be viewed as a compromise between store-and-forward switching and cut-through switching. The reason fragment-free switching stores only the first 64 bytes of the frame is that most network errors and collisions occur during the first 64 bytes. Fragment-free switching tries to enhance cut-through switching by performing a small error check on the first 64 bytes of the frame to ensure that a collision has not occurred before forwarding the frame. Fragment-free switching is a compromise between the high latency and high integrity of store-and-forward switching, and the low latency and reduced integrity of cut-through switching.

Some switches are configured to perform cut-through switching on a per-port basis until a user-defined error threshold is reached and then they automatically change to store-and-forward. When the error rate falls below the threshold, the port automatically changes back to cut-through switching.


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