3Com Switch 4200G Configuration Guide

Introduction to QoS 219
For example, if the device A sends packets to the device B. The device B will perform
TP on packets from the device A to drop the packets beyond the specification.
In order to avoid meaningless packet loss, you can perform TS on the packets on the
egress of the device A and cache the packets beyond the TP specification in the device
A. When the next packets can be sent, the packets cached in the buffer queues will
be taken out and sent. In this way, all the packets sent to the device B conforms to
the traffic specification of the device B.
Redirect You can re-specify the forwarding port of packets as required by your own QoS policy.
Queue Scheduling When the network is congested, the problem that many packets compete for
resources must be solved, usually in the way of queue scheduling.
In the following section, SP (Strict-Priority) queues, WRR (Weight Round Robin)
queues and SDWRR (Shaped Deficit WRR) queues are introduced.
1 SP queue
Figure 69 Diagram for SP queues
SP queue-scheduling algorithm is specially designed for critical service applications.
An important feature of critical services is that they demand preferential service in
congestion in order to reduce the response delay. Assume that there are 8 output
queues on the port and the preferential queue classifies the 8 output queues on the
port into 8 classes, which are queue7, queue6, queue5, queue4, queue3, queue2,
queue1, and queue0. Their priorities decrease in order.
In the queue scheduling, SP sends packets in the queue with higher priority strictly
following the priority order from high to low. When the queue with higher priority is
empty, packets in the queue with lower priority are sent. You can put critical service
packets into the queues with higher priority and put non-critical service (such as
e-mail) packets into the queues with lower priority. In this case, critical service packets
are sent preferentially and non-critical service packets are sent when critical service
groups are not sent.
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queueDequeue
queue 7
queue 6
queue 5
queue 4
queue 3
queue 2
queue 1
queue 0
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queueDequeue
queue 7
Packets sent via this
interface
high priority
Low priority
Classify
Packets sent
Sending queueDequeue
queue 7
queue 6
queue 5
queue 4
queue 3
queue 2
queue 1
queue 0