All platforms use store-and-forward by default. The platforms are also capable of supporting cut-through forwarding to reduce latency. Store-and-forward switching requires the complete receipt of a packet prior to transmitting it out the interface. The packet is stored in its entirety in packet memory and can be validated via the frame CRC by the switch prior to forwarding it to the next hop.
Cut-through switching allows the switch to begin transmitting a packet before its entire contents have been received thereby reducing the overall forwarding latency for large packet sizes.
Cut-through forwarding mode is supported only on the 40G ports of the Summit X770, ExtremeSwitching X670-G2, X870, and X690 series switches; and 100G ports on ExtremeSwitching X870 series switches.
Error packets may be forwarded when using cut-through mode. These packets need to be detected and discarded by one of the downstream switches, routers, or the ultimate end station.
Cut-through mode cannot be achieved when switching a packet internally from a low-speed front-panel port (1G or 10G) to a higher-speed fabric port. In this case, store-and-forward switching will automatically be used. However, cut-through switching can be used when switching between equal speed ports or from a higher-speed interface to a lower-speed interface.
Store-and-forward is used for packets that are switched to multiple egress ports in scenarios such as VLAN (Virtual LAN) flooding and multicast.
Store-and-forward is used whenever the egress interface is congested including when QoS (Quality of Service) rate shaping is in effect.
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