What is "Cut-through", "store and forward" and "fragment free" switching methods?
Switching methods forward layer 2 frames in different ways
- Cut through
- Store and forward
- Fragment free
Cut-through switching reads only the the destination part of mac header which means that the switch will begin forwarding without reading the other parts of the header, this makes this type of switching very fast with low latency but compromises on checking the CRC part of a header which might lead to the switch forwarding corrupt packets. Certain cisco devices can read more fields in the L2/mac header in a cut-through switching mode.
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 beenreceived 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 isslower than the switching speed in cut-through switching mode.
Source: b-cisco-nexus-9000-nx-os-layer-2-switching-configuration-guide-92x_chapter_01000.pdf
need to double check this:
- Cut through, ironically the destination part of the mac header is the LAST field type which means (?) that the switch still has to process most of the frame before forwarding...