Qos Output Scheduling; Configuring The Combination Queue - D-Link Xstack DGS-3600 Series User Manual

The Scheduling Mechanism has the following parameters.
Parameter
Strict
Weight fair
Click Apply to implement changes made.

QoS Output Scheduling

QoS can be customized by changing the output scheduling used for
the hardware classes of service in the Switch. As with any changes
to QoS implementation, careful consideration should be given to
how network traffic in lower priority classes of service is affected.
Changes in scheduling may result in unacceptable levels of packet
loss or significant transmission delay. If choosing to customize this
setting, it is important to monitor network performance, especially
during peak demand, as bottlenecks can quickly develop if the QoS
settings are not suitable. In the QoS folder click QoS Output
Scheduling, to view the screen shown below.
The following values may be assigned to the QoS classes to set the scheduling.
Parameter Description
Max. Packets Specifies the maximum number of packets the above specified hardware priority class of service
will be allowed to transmit before allowing the next lowest priority queue to transmit its packets.
A value between 0 and 15 can be specified.
Click Apply to implement changes made.

Configuring the Combination Queue

Utilizing the QoS Output Scheduling Configuration window shown above, the xStack DGS-3600 switch series can implement a
combination queue for forwarding packets. This combination queue allows for a combination of strict and weight-fair (weighted
round-robin "WRR") scheduling for emptying given classes of service. To set the combination queue, enter a 0 for the Max
Packets entry of the corresponding priority classes of service listed in the window above. Priority classes of service that have a 0
in the Max Packet field will forward packets with strict priority scheduling. The remaining classes of service, that do not have a 0
in their Max Packet field, will follow a weighted round-robin (WRR) method of forwarding packets — as long as the priority
classes of service with a 0 in their Max Packet field are empty. When a packet arrives in a priority class with a 0 in its Max
Packet field, this class of service will automatically begin forwarding packets until it is empty. Once a priority class of service
with a 0 in its Max Packet field is empty, the remaining priority classes of service will reset the weighted round-robin (WRR)
cycle of forwarding packets, starting with the highest available priority class of service. Priority classes of service with an equal
level of priority and equal entries in their Max Packet field will empty their fields based on hardware priority scheduling. The
Max Packet parameter allows the maximum number of packets a given priority class of service can transmit per weighted round-
robin (WRR) scheduling cycle to be selected. This provides for a controllable CoS behavior while allowing other classes to empty
as well. A value between 0 and 15 packets can be specified per priority class of service to create the combination queue.
The example window below displays an example of the combination queue where Class-1 will have a strict priority for emptying
its class, while the other classes will follow a weight fair scheduling.
xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch
Description
The highest class of service is the first to process traffic. That is, the highest class of service
will finish before other queues empty.
Use the weighted round-robin (WRR) algorithm to handle packets in an even distribution in
priority classes of service.
Figure 9- 4. QoS Output Scheduling window
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