Configuring Switching Modes

Information About Switching Modes

The switching mode determines whether the switch begins forwarding the frame as soon as the switch has read the destination details in the packet header or waits until the entire frame has been received and checked for cyclic redundancy check (CRC) errors before forwarding them to the network.

The switching mode is applicable to all packets being switched or routed through the hardware and can be saved persistently through reboots and restarts.

The switch operates in either of the following switching modes:

Cut-Through Switching Mode

Cut-through switching mode is enabled by default. Switches operating in cut-through switching mode start forwarding the frame as soon as the switch has read the destination details in the packet header. A switch in cut-through mode forwards the data before it has completed receiving the entire frame.

The switching speed in cut-through mode is faster than the switching speed in store-and-forward switching mode.

Store-and-Forward Switching Mode

When store-and-forward switching is enabled, the switch checks each frame for cyclic redundancy check (CRC) errors before forwarding them to the network. Each frame is stored until the entire frame has been received and checked.

Because it waits to forward the frame until the entire frame has been received and checked, the switching speed in store-and-forward switching mode is slower than the switching speed in cut-through switching mode.

Guidelines and Limitations for Switching Modes

Consider the following guidelines and limitations for each of the switching modes:

Cut-Through Switching Mode Guidelines and Limitations

  • show commands with the internal keyword are not supported.

  • When an FCS error is discovered on a cut-through switch, the FCS error packets may not be dropped, as the packet transmission out of the egress port may have already been in progress. When a cut-through switch detects a CRC packet after it has already been sent out of an egress port, the CRC field of the packet is stomped and generally marked as an output error on the egress interface. The packet is then dropped at the next store-and-forward network node, or delivered to the destination if all nodes along the path are cut-through switching as well.

  • Packets with FCS errors are not mirrored if SPAN is configured.

  • Beginning in 7.0(3)I1(2), cut-through switching is supported on the Cisco Nexus 9500 Series switch with the 9636PQ line card.

Store-and-Forward Switching Mode Guidelines and Limitations

  • show commands with the internal keyword are not supported.

  • Packets with FCS errors are dropped.

  • Packets with FCS errors are not mirrored if SPAN is configured.

  • The CPU port always operates in store-and-forward mode. Any packets forwarded to the CPU with FCS errors are dropped.

  • Store-and-forward mode activates automatically for a port when the switch identifies that the port is oversubscribed and the ingress rate is greater than the switching capacity of the egress port. For example, when the port ingress rate is 10 gigabit and the switching capacity of the egress port is 1 gigabit.


    Note

    The global configuration does not change, even if store-and-forward mode is activated for an oversubscribed port.

Default Settings for Switching Modes

Cut-through switching is enabled by default.

Configuring Switching Modes

Enabling Store-and-Forward Switching


Note

Enabling store-and-forward switching mode might impact your port-to-port switching latency.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config) # switching-mode store-forward
  3. (Optional) switch(config)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config) # switching-mode store-forward

Enables store-and-forward switching mode.

Step 3

(Optional) switch(config)# copy running-config startup-config

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to enable store-and-forward switching: 

switch# configure terminal
switch(config) # switching-mode store-forward
switch(config) #

Reenabling Cut-Through Switching

Cut-through switching is enabled by default. To reenable cut-through switching, use the no form of the switching-mode store-forward command.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config) # no switching-mode store-forward
  3. (Optional) switch(config)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config) # no switching-mode store-forward

Disables store-and-forward switching mode. Enables cut-through switching mode.

Step 3

(Optional) switch(config)# copy running-config startup-config

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to reenable cut-through switching: 

switch# configure terminal
switch(config) # no switching-mode store-forward
switch(config) #