Users Guide

● Reserved buffer—The system reserves a dedicated amount of buffer to a port or a priority group (at ingress) and a port or a
queue (at egress).
● Shared buffer—Is the total available buffer space minus the reserved buffer space. Shared buffer is used for CPU control
traffic and is dynamically allocated to the ports when memory space is needed.
● Alpha value—Is a configurable value from 0 to 10 that determines the dynamic shared buffer threshold, and maintains
dynamic buffer space during congestion events.
● Xoff threshold (transmit off)—When the system reaches the Xoff threshold, to prevent traffic loss, the system pauses and
does not accept any further packets.
● Xon threshold (transmit on)—When the system reaches the Xon threshold, the system resumes and accepts the packets.
For example, when all ports are allocated as reserved buffers from the lossy (default) pool, the remaining buffers in the lossy
pool are shared across all ports, except the CPU port.
When you enable priority flow control (PFC) on the ports, all the PFC-enabled queues and priority-groups use the buffers from
the lossless pool.
You must use the network QoS policy type to configure PFC on the ports.
OS10 dedicates a separate buffer pool for CPU traffic. All default reserved buffers for the CPU port queues are from the CPU
pool. The remaining buffers are shared across all CPU queues. You can modify the buffer settings of CPU queues.
You can configure the size of the CPU pool using the control-plane-buffer-size command.
OS10 allows configuration of buffers per priority-group and queue for each port.
Buffer-usage accounting happens for ingress packets on ingress pools and egress packets on egress pool. You can configure
ingress-packet buffer accounting per priority-group and egress-packet buffer accounting per queue level.
Configuration notes
Dell EMC PowerSwitch S4200-ON Series:
● Provisioning LLFC is not supported when deep buffer mode is enabled.
● Stop the traffic before applying or modifying the LLFC configuration.
Configure ingress buffer
By default, all traffic classes map to the default priority group (PG) 7 for ingress buffers. The buffer reservation is based on the
default priority group ID 7. All buffers are part of the default pool and all ports share buffers from the default pool. When you
configure a network qos policy map, a new priority group is created for which buffers are assigned from the lossless pool. The
rest of the traffic classes that are not mapped to any PFC-related PGs, use the default buffer.
The reserved buffer size is 9360 bytes for the speed of 10G, 25G, 40G, 50G, and 100G. The supported speed varies for
different platforms.
Table 140. Maximum buffer size
Platforms Max buffer size
S4000 12 MB
S6010–ON, S4048–ON 16 MB
S4100-ON Series 12 MB
S4200-ON Series 6 GB
S5200-ON Series 32 MB
Z9100–ON 16 MB
Z9264F-ON 42 MB
The following table lists the values allocated for the default ingress buffers on the S4100-ON series platform. These values
may differ for different platforms and speeds. Use the show qos ingress buffers command to view the default ingress
buffers on your switch.
Table 141. Default ingress buffers on the S4100-ON series platform
Quality of service 1549