Netis ST3326(ST-3302) [88/118] Configuring queue scheduling

corresponding to the re-marked local precedence.

 If IP precedence or DSCP marking is configured, the traffic will be marked with new IP

precedence or DSCP precedence.

Configuring Queue Scheduling

When the network is congested, the problem that many packets compete for resources must be

solved, usually through queue scheduling.

In the following section, strict priority (SP) queues, weighted round robin (WRR), and SP+WRR

(High Queue-WRR) queues are introduced.

1) SP queuing

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 four output queues on the port and the

preferential queue classifies the four output queues on the port into four classes, which are

queue 3, queue 2, queue 1, and queue 0. Their priorities decrease in order.

In 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.

The disadvantage of SP queue is that: if there are packets in the queues with higher priority for a

long time in congestion, the packets in the queues with lower priority will be “starved” because

they are not served.

2) WRR queuing

WRR queue-scheduling algorithm schedules all the queues in turn and every queue can be

assured of a certain service time. Assume there are four output queues on a port. WRR

configures a weight value for each queue, which is w3, w2, w1, and w0 for queue 3 through

queue 0. The weight value indicates the proportion of obtaining resources. On a 100 M port,

configure the weight value of WRR queue-scheduling algorithm to 5, 3, 1, and 1 (corresponding

to w3, w2, w1, and w0 in order). In this way, the queue with the lowest priority can get 10 Mbps

bandwidth (100-Mbps × 1/ (5 + 3 + 1 + 1)) at least, and the disadvantage of SP queue-scheduling

that the packets in queues with lower priority may not get service for a long time is avoided.

Another advantage of WRR queue is that: though the queues are scheduled in order, the service

time for each queue is not fixed; that is to say, if a queue is empty, the next queue will be

scheduled. In this way, the bandwidth resources are made full use.

3) SP+WRR queuing

SP+WRR is an improvement over WRR. Assume there are four priority queues on a port and

queue 3 allocated with the highest priority, the switch will ensure that this queue get served first