Brocade Communications Systems 53-1002163-02 manual Traffic class scheduling policy

11 Scheduling

Deficit Weighted Round Robin (DWRR) is an improved version of WRR. DWRR remembers the excess used when a queue goes over its bandwidth allocation and reduces the queue's bandwidth allocation in the subsequent rounds. This way the actual bandwidth usage is closer to the defined level when compared to WRR.

Traffic class scheduling policy

The traffic classes are numbered from 0 to 7; higher numbered traffic classes are considered higher priority. Fabric OS v7.0.0 provides full flexibility in controlling the number of SP-to-WRR queues. The number of SP queues is specified in N (SP1 through 8), then the highest priority traffic classes are configured for SP service and the remaining eight are WRR serviced. Table 27 describes the set of scheduling configurations supported.

When you configure the QoS queue to use strict priority 4 (SP4), then traffic class 7 will use SP4, traffic class 6 will use SP3, and so on down the list. You use the strict priority mappings to control how the different traffic classes will be routed in the queue.

TABLE 27	Supported scheduling configurations
Traffic Class	SP0	SP1	SP2	SP3	SP4	SP5	SP6	SP8
7	WRR8	SP1	SP2	SP3	SP4	SP5	SP6	SP8
6	WRR7	WRR7	SP1	SP2	SP3	SP4	SP5	SP7
5	WRR6	WRR6	WRR6	SP1	SP2	SP3	SP4	SP6
4	WRR5	WRR5	WRR5	WRR5	SP1	SP2	SP3	SP5
3	WRR4	WRR4	WRR4	WRR4	WRR4	SP1	SP2	SP4
2	WRR3	WRR3	WRR3	WRR3	WRR3	WRR3	SP1	SP3
1	WRR2	WRR2	WRR2	WRR2	WRR2	WRR2	WRR2	SP2
0	WRR1	WRR1	WRR1	WRR1	WRR1	WRR1	WRR1	SP1

Figure 10 shows that extending the frame scheduler to a hybrid SP+WRR system is fairly straightforward. All SP queues are considered strictly higher priority than WRR so they are serviced first. Once all SP queues are drained, then the normal WRR scheduling behavior is applied to the non-empty WRR queues.

FIGURE 10 Strict priority and Weighted Round Robin scheduler

Scheduling the QoS queue

To specify the schedule to use, perform the following steps from privileged EXEC mode. 1. Enter global configuration mode.

switch#configure terminal

118	Converged Enhanced Ethernet Administrator’s Guide
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