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.
For the Summit X770 and ExtremeSwitching X870, both 40G and 10G ports support cut-through switching mode.
Summit X670-G2 supports cut-through switching. ExtremeSwitching X690 supports cut-through switching on 40G ports only.
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.
Print
this page
Email this topic
Feedback
View PDF
Download EPUB