Planet Technology Planet Intelligent Gigabit Ethernet Stackable/Routing Switch WGSW-2402A - Manual (Page 162)

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5.15.1.5 Weighted Fair Queuing
In some environments – for example, in an environment in which delay assurances are not required,
but precise bandwidth partitioning on small time scales is essential, WFQ may be preferable to a
delay-bounded scheduling discipline. The Switch provides the user with a WFQ option with the
understanding that delay assurances can not be provided if the incoming traffic pattern is uncontrolled.
In WFQ mode, though we do not assure frame latency, the Switch still retains a set of dropping rules
that helps to prevent congestion and trigger higher level protocol end-to-end flow control.
As before, when strict priority is combined with WFQ, the Switch do not have special dropping rules for
the strict priority queues, because the input traffic pattern is assumed to be carefully controlled at a
prior stage. However, the Switch do indeed drop frames from SP queues for global buffer
management purposes. In addition, queue P0 for a 10/100 port (and queues P0 and P1 for a Gigabit
port) are treated as best effort from a dropping perspective, though they still are assured a percentage
of bandwidth from a WFQ scheduling perspective. What this means is that these particular queues
are only affected by dropping when the global buffer count becomes low.
5.15.1.6 Shaper and DiffServ Expedited Forwarding
Although traffic shaping is not a primary function of the Switch, it does implement a shaper for
expedited forwarding (EF). The goal in shaping is to control the peak and average rate of traffic exiting
the Switch. Shaping is limited to the Gigabit ports only, and only to class P6 (the second highest
priority). This means that class P6 will be the class used for EF traffic. If shaping is enabled for P6, then
P6 traffic must be scheduled using strict priority. With reference to Table 5-4, only the middle two QoS
configurations may be used.
Maximum rate and average rate is multiple of 6.25%. If the maximum rate is 50% and average rate is
25%, shaped traffic will exit the Switch at a rate always less than 500 Mbps, and averaging no greater
than 250 Mbps.
Also, when shaping is enabled, it is possible for a P6 queue to explode in length if fed by a greedy
source. The reason is that a shaper is by definition not work-conserving; that is, it may hold back from
sending a packet even if the line is idle. Though the Switch does have global resource management, it
does nothing to prevent this situation locally. We assume SP traffic is policed at a prior stage to the
Switch.
5.15.1.7 Rate Control
The Switch provides a rate control function on its 10/100 ports. This rate control function applies to the
outgoing traffic aggregate on each 10/100 port. It provides a way of reducing the outgoing average rate