Users Guide

Low Latency Modes
Low latency describes a system network that processes a high volume of data messages with minimal delay (latency). These
networks support operations that require near real-time access to rapidly changing data. Use Low Latency mode to reduce the
switching latency for timing-critical applications such as storage networks. By default, Low Latency mode is not enabled in OS10
switches. To achieve low latency, only the Memory Management Unit (MMU) Cut-Through (CT) mode is enabled.
● Low Latency modes include bypass paths in different blocks within the ingress and egress switching pipeline.
● MMU CT mode switches send the packets to the destination port without buffering the entire packet in the MMU buffer.
Cut-through switching mode
CT switching offers low-latency performance for SCSI traffic. Use CT switching in packet-switching systems. The switch
forwards packets or frames to its destination immediately after the destination address is processed without waiting to receive
the entire data.
The egress scheduler block in the NPU pipeline schedules the packet to transmit out after the first cells of packet arrive.
However, egress scheduler falls back to Store and Forward (SF) mode if the conditions are not met for CT transmission, even
though you configure the switch in CT mode.
The following conditions must be met for the switch to operate in CT mode. If these conditions are not met, the switch stays
in SF mode, irrespective of the configured value. For Multicast packets, all the destination ports must satisfy the following
conditions:
1. The source and destination port speed must be within the configured range. The range can be same speed ports or
fast-to-slow speed ports. For example 10G to 10G, 40G to 40G, 40G to 10G, 400G to 100G. For more information, see
Restrictions and Limitations.
2. The destination port must not experience a back-pressure due to Priority Flow Control (PFC) or pause frames.
3. Do not overlap the destination port. Multiple ingress ports must not send packets to the same destination port. Similarly, one
ingress port must not send multiple copies of a packet to the same port; for example, unicast or mirror copy.
4. The queue of the destination port must not have packets waiting for transmission in SF mode.
5. If the port-max shaper is configured on the egress port, outgoing packets rate on the egress port must be in the configured
range. The transmitted packet rate is less than the configured maximum peak rate.
6. If the policer configuration is enabled on the ingress port in CT Switching mode and if the incoming packet rate is higher
than the peak information rate, the excess traffic drops at ingress. The outgoing traffic which abides in the policer
configuration transmits in CT mode.
7. If the source-to-destination port path of the packet is in CT mode, no other source ports can queue their packets to the
same destination port.
8. CT mode switching is not allowed to the CPU port.
9. CT mode is not allowed to the Loopback port.
10. CT mode is not allowed to the management port.
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System management