System information

10-27
Port Status and Basic Configuration
QoS Passthrough Mode
QoS Passthrough Mode
QoS Passthrough mode is designed to enhance the performance of line-rate
traffic transfers through the switch. This feature should only be used in
environments where Quality of Service (QoS) is not of major importance, but
where lossless data transfers are key. This command disables any discrimina-
tion of QoS queues for traffic, consolidating frame buffer memory to provide
line-rate flows with no loss of data.
General Operation
The port buffering design for the switch has been optimized for gigabit-to-
gigabit traffic flows. For this reason, some flows from Gigabit-to-100Base or
even 100Base-to-10Base may not perform as well as would be expected. The
QoS Passthrough mode enhancement can provide a significant performance
improvement for high-bandwidth traffic flows through the switch, particularly
when running traffic flows from 1000Base to either 100Base or 10Base con-
nections.
QoS Pass-Through mode is set to “optimized” by default. If it has been set to
“typical”, you can re-enable it to “optimized” using the CLI command
qos-passthrough-mode (in the config context) followed by write memory and
rebooting the switch.
QoS Passthrough mode, when set to “optimized”, results in the following
general changes to switch operation:
■ Alters the switch's default outbound priority queue scheme from four
queues (low, normal, medium, and high), to two queues (normal & high).
■ Optimizes outbound port buffers for a two-queue scheme.
■ All packets received with an 802.1p priority tag of 0 to 5 (low, normal, or
medium priorities), or tagged by the switch's QOS feature, will be serviced
by the (now larger) “normal” priority queue.
■ All packets received with an 802.1p priority tag of 6 or 7 (high priority),
or tagged by the switch's QoS feature, will be serviced by the “high”
priority queue.
■ High priority packets sourced by the switch itself, such as Spanning Tree
frames, will be serviced in the “high” priority queue.
■ Any 802.1p tagging on a received packet, or any tag added to a received
packet by the switch via its QoS configuration, will be preserved as it is
transmitted from the switch.