System Description
H3C S3600 Series Ethernet Switches
Chapter 3 Software Features
3-11
I. SP
Classify
Packets to be sent
via this interface
Packets left
the interface
High
Medium
Normal
Low
Dequeue
Queue
Figure 3-1
SP
The SP mechanism applies to key services that are delay sensitive and must have
priority when congestion occurs. In SP, packets are assigned to four queues, namely,
high-priority queue, medium-priority queue, normal priority queue, and low-priority
queue (queue 3, 2, 1, and 0 respectively) with decreasing priorities.
SP schedules the packets in a strict priority order. It sends the packets in a queue only
when the queue with a higher priority is empty. By putting the key service packets in the
high priority queues, you can ensure that they can always be served first. At the same
time the common service packets can be put in the low priority queues and transmitted
when there are no key service packets waiting for transmission.
If congestion occurs and the high priority queues are occupied for a long time, however,
the packets in the lower-priority queues are “starved” before obtaining services.
II. WRR
In WRR, there are four or eight egress queues on each port. The packets in different
queues are processed in turn, so that every queue is assigned some time of service. If
there are four queues on a port, they are each assigned a weight for obtaining
resources: w3, w2, w1, and w0 respectively. On a 100 Mbps port for example, you can
assigns the weights 50, 30, 10, 10 to the four queues with w3, w2, w1 and w0. Thus the
lowest-priority queue can be guaranteed of a minimum bandwidth of 10 Mbps. This
avoids the case that the packets in the low priority queues cannot be served, as in SP.
More than that, WRR assigns service time slices flexibly to every queue. When a queue
is empty, the next one is processed immediately. Thus it makes a full use of the
bandwidth resources.
III. WFQ
WFQ classifies packets by flow. The packets with the same source IP address,
destination IP address, source port number, destination port number, protocol, or TOS
are regarded a flow and are assigned to the same queue. When WFQ dequeues
