xStack
®
DGS-3400 Series Layer 2 Gigabit Ethernet Managed Switch
180
Configuring the Combination Queue
Utilizing the
QoS Output Scheduling
window shown above, the xStack
®
DGS-3400 Series can implement a combination queue
for forwarding packets. This combination queue allows for a combination of strict and weight-fair (weighted round-robin, or
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 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 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 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.
Figure 4 - 5 QoS Output Scheduling window – Combination queue example
