Configuration Guide User guide

FastIron Configuration Guide 1987
53-1002494-02
Scheduling QoS information
Scheduling QoS information
Scheduling is the process of mapping a packet to an internal forwarding queue based on its QoS
information, and servicing the queues according to a mechanism.
Scheduling for the SX-FI48GPP module
The SX-FI48GPP module supports scheduling at the front-end and back-end NP. If egress
congestion occurs at the front-end NP of the SX-FI48GPP module, scheduling is based on four
queues instead of eight. For more information on default configuration for 8 to 4 queue mapping,
refer to Table 334 on page 1972. If egress congestion occurs at the back-end of the SX-FI48GPP
module, then scheduling is based on eight queues. When SX- FI48GPP ports are running at a
reduced speed (100 Mbps or 10 Mbps), egress congestion usually occurs at the front-end NP.
QoS queuing methods
The following QoS queuing methods are supported in all IronWare releases for the FastIron devices:
• Weighted Round Robin (WRR) – WRR ensures that all queues are serviced during each cycle. A
WRR algorithm is used to rotate service among the eight queues on the FastIron devices. The
rotation is based on the weights you assign to each queue. This method rotates service among
the queues, forwarding a specific number of packets in one queue before moving on to the
next one.
NOTE
In stacking mode, the qosp7 queue is reserved as Strict Priority (SP) under weighted queuing.
Attempts to change the qosp7 setting will be ignored.
WRR is the default queuing method and uses a default set of queue weights.
The number of packets serviced during each visit to a queue depends on the percentages you
configure for the queues. The software automatically converts the percentages you specify into
weights for the queues.
NOTE
Queue cycles on the FastIron devices are based on bytes. These devices service a given
number of bytes (based on weight) in each queue cycle. FES and BI/FI queue cycles are based
on packets. The bytes-based scheme is more accurate than a packets-based scheme if
packets vary greatly in size.
• Strict Priority (SP) – SP ensures service for high-priority traffic. The software assigns the
maximum weights to each queue, to cause the queuing mechanism to serve as many packets
in one queue as possible before moving to a lower queue. This method biases the queuing
mechanism to favor the higher queues over the lower queues.
For example, strict queuing processes as many packets as possible in qosp3 before processing
any packets in qosp2, then processes as many packets as possible in qosp2 before processing
any packets in qosp1, and so on.
• Hybrid WRR and SP – This configurable queueing mechanism combines both the SP and WRR
mechanisms. The combined method enables the Brocade device to give strict priority to
delay-sensitive traffic such as VoIP traffic, and weighted round robin priority to other traffic
types.