- Preface
- New and Changed Information for this Release
- Overview
- Configuring Ethernet Interfaces
- Configuring VLANs
- Configuring Private VLANs
- Configuring Access and Trunk Interfaces
- Configuring Switching Modes
- Configuring Rapid PVST+
- Configuring Multiple Spanning Tree
- Configuring STP Extensions
- Configuring LLDP
- Configuring the MAC Address Table
- Configuring IGMP Snooping
- Configuring Traffic Storm Control
- Index
Contents
- Configuring Switching Modes
- Information About Switching Modes
- Guidelines and Limitations for Switching Modes
- Licensing Requirements for Switching Modes
- Default Settings for Switching Modes
- Configuring Switching Modes
- Enabling Store-and-Forward Switching
- Reenabling Cut-Through Switching
- Feature History for Switching Modes
Configuring Switching Modes
This chapter contains the following sections:
- Information About Switching Modes
- Guidelines and Limitations for Switching Modes
- Licensing Requirements for Switching Modes
- Default Settings for Switching Modes
- Configuring Switching Modes
- Feature History for 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
Packets with frame check sequence (FCS) errors are dropped. For example: - For the Cisco Nexus 3064PQ platform, packets smaller than or equal to 768 bytes are dropped.
- For Cisco Nexus 3016, 3064E, 3064X, and 3048 platforms, packets smaller than or equal to 560 bytes are dropped.
- For the Cisco Nexus 3064PQ platform, packets larger than 769 or equal to bytes are forwarded.
- For 3016, 3064E, 3064X, and 3048 platforms, packets larger than or equal to 561 bytes are forwarded.
- Packets with FCS errors are not mirrored if SPAN is configured.
Store-and-Forward Switching Mode Guidelines and Limitations
- Packets that are smaller than the maximum transmission unit (MTU) size configured on the switch 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.
NoteThe global configuration does not change, even if store-and-forward mode is activated for an oversubscribed port.
Licensing Requirements for Switching Modes
Cut-through switching mode and store-and-forward switching modes do not require licenses. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide.
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. |
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.
This example shows how to reenable cut-through switching:
switch# configure terminal switch(config) # no switching-mode store-forward switch(config) #
Feature History for Switching Modes
Feature Name |
Releases |
Information |
|---|---|---|
Store-and-Forward Switching |
5.0(3)U3(1) |
This feature was introduced. |