Default Switching Mode

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, both 40G and 10G ports support store-and-forward switching mode. On the X770, cut-through switching mode is only supported on 40G ports, and is not supported on 10G ports.

Summit X670-G2 also support cut-through switching.

The following limitations apply to the cut-through switching feature:
  • 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.

In some circumstances, store-and-forward is automatically used. Following are examples:
  • 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 flooding and multicast.

  • Store-and-forward is used whenever the egress interface is congested including when QoS rate shaping is in effect.