LAN Switch Types


LAN switch types decide how a frame is handled when it’s received on a switch port. Latency—the time it takes for a frame to be sent out an exit port once the switch receives the frame—depends on the chosen switching mode. There are three switching modes:

  • Cut-through

  • FragmentFree

  • Store-and-forward

Cut-Through (Real-Time)

When in cut-through or real-time mode, the switch waits for the destination hardware address to be received before it looks up the destination address in the MAC filter table. With the cut-through switching method, the LAN switch copies only the destination address (the first six bytes following the preamble) onto its onboard buffers. That done, it then looks up the hardware destination address in the MAC switching table, determines the outgoing interface, and proceeds to forward the frame toward its destination.

A cut-through switch really helps to reduce latency because it begins to forward the frame as soon as it reads the destination address and determines the outgoing interface. And after it determines the destination port, the subsequent frames are immediately forwarded out through it.

With some switches, you get an extra supercool feature: the flexibility to perform cut-through switching on a per-port basis until a user-defined error threshold is reached. At the point that threshold is attained, the ports automatically change over to store-and-forward mode so they stop forwarding the errors. And, when the error rate on the port falls back below the threshold, the port automatically changes back to cut- through mode.

FragmentFree (Modified Cut-Through)

FragmentFree, or modified cut-through, is the default switching mode for the Catalyst 1900 switch. In this mode, the switch checks the first 64 bytes of a frame before forwarding it for fragmentation, thus guarding against possible collisions. FragmentFree switching is a modified form of cut-through switching, in which the switch waits for the collision window (64 bytes) to pass before forwarding. This is because if a packet has an error, it almost always occurs within the first 64 bytes. It means each frame is checked into the data field to make sure no fragmentation has occurred.

FragmentFree switching mode provides better error checking than the cut-through mode with practically no increase in latency. It’s the default switching method for the Catalyst 1900 switches.

Store-and-Forward

Store-and-forward switching is Cisco’s primary LAN switching method. When in store-and-forward mode, the LAN switch copies the entire frame onto its onboard buffers and then computes the cyclic redundancy check (CRC). Because it copies the entire frame, latency through the switch varies with frame length.

The frame is discarded if it contains a CRC error, if it’s too short (less than 64 bytes including the CRC), or if it’s too long (more than 1,518 bytes, including the CRC). If the frame doesn’t contain any errors, the LAN switch looks up the destination hardware address in its forwarding or switching table to find the correct outgoing interface. When it does, out goes the frame toward its destination. This is the mode used by the Catalyst 5000 series switches, and you can’t modify it.

With store-and-forward switching, the complete data frame is received on the switch’s buffer, a CRC is run, and then the switch looks up the destination address in the MAC filter table. Figure 2.10 delimits the different points where the switching mode takes place in the frame.

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Figure 2.10: Different switching modes within a frame

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Real World Scenario—Practical Configuration of Switching Modes

Many of the Cisco switches (the Catalyst 2950 series, for example) can be configured for multiple modes of switching to meet network demands. For instance, you can configure a switch port to use cut-through switching for lower latency and specify a threshold value at which the switch changes to store-and-forward switching if a high rate of errors is detected or what’s called a collision window passes. (A collision window is the maximum amount of time required to detect an error.) You can also specify the type of traffic that should be processed using cut-through or store-and-forward switching. For example, you can configure unicast traffic to use cut- through switching and broadcast traffic to use only store-and-forward switching. There are typically different time-sensitive considerations for unicast and broadcast traffic. Multicast traffic can also be configured for either type of switching. Depending upon the type of multicast traffic, video, IP/TV, and synchronous learning, there are different considerations for permitted latency.

Configuring the different switching modes on a Cisco switch is relatively easy when using the command line. For example, on the Catalyst 2950 series switches, if you enter switching-type ?, two options are available: fragment- free and store-and-forward. You can type switching-type fragment-free or switching-type store-and-forward to set either switching mode globally on that switch. The third switching-type option, cut-through, does not display since it is the default.

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CCDA. Cisco Certified Design Associate Study Guide
CCDA: Cisco Certified Design Associate Study Guide, 2nd Edition (640-861)
ISBN: 0782142001
EAN: 2147483647
Year: 2002
Pages: 201

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