Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring QoS (Rev. 03)
Overview
© 2008 Foundry Networks, Inc.
Page 8 of 62
Benefits of SP Queuing
SP provides absolute preferential treatment to high priority traffic, ensuring that mission-critical
traffic traversing various WAN links gets priority treatment. In addition, SP provides a faster
response time than do other methods of queuing.
Weighted Round Robin (WRR)
In this scheduling method, a weighting factor for each queue determines how many bytes of data
the system delivers from the queue before it moves on to the next queue. The WRR mechanism
cycles through the queues. For each queue, packets are sent until the number of bytes transmitted
exceeds the bandwidth determined by the queue weighting factor, or the queue is empty. Then the
WRR mechanism moves to the next queue. If a queue is empty, the router will send packets from
the next queue that has packets ready to send.
Note that if a packet length exceeds the queue allowed bandwidth, the packet is still transmitted
during its time slot, but its quota is overdrawn so that on the next time slot it receives a smaller
allotment. This mechanism guarantees a minimum bandwidth to each queue, but allows the
minimum to be exceeded if one or more of the port other queues are idle. However, when all the
queues are loaded each is limited to its maximum bandwidth according to its assigned weight - no
queue achieves more than a predetermined proportion of overall capacity when the line is under
stress.
The weighting factors are specified as relative percentages. The values for all the queues must be
positive, in the range 1-15.
Relative percentages are calculated by packet counts.
Figure 5
shows how WRR queuing behaves in a four-queue architecture.
Figure 5: Weighted Round Robin Queuing
Benefits of WRR Queuing
WRR scheduling prevents the low-priority queues from being completely neglected during periods
of high-priority traffic. By using this scheduling, low-priority queues have the opportunity to
transmit packets even though the high-priority queues are not empty.