Riverstone Networks WICT1-12 8.4 TRAFFIC MANAGEMENT

8-8 Riverstone Networks RS Switch Router User Guide Release 8.0

Traffic Management ATM Configuration Guide

8.4 TRAFFIC MANAGEMENT

The ATM line cards provide different methods for managing traffic. On the ATM multi-rate line card you can use the

following QoS policies to control ATM traffic: Strict Priority, Weighted Fair Queueing (WFQ), or WFQ with Strict

Priority.

On the ATM OC-12 line card you can prioritize traffic by configuring virtual channel (VC) groups. Each VC within a

VC group can be assigned one (or more) of four internal levels: low, medium, high, and control. These levels prioritize

the separate VCs, and as a result prioritize the traffic passing through the separate VCs within the VC group.

You can use the atm set vcl-qos command to set the following QoS policies on the ATM multi-rate line card:

•Strict Priority (Default)

Separate buffer space is allocated to each of the following four queues: control, high, medium and low. Buffered

traffic is forwarded in the following order: traffic in the control queue is forwarded first, followed by traffic in the

high queue, medium queue, and finally, the low queue. When using strict priority, no control packets are dropped

if the rate of the control packets is less then the VC's rate. But if the rate of control packets exceeds the VC's rate,

then some control packets must be dropped.

This policy ensures that critical traffic reaches its destination even if the exit ports for the traffic are experiencing

greater-than-maximum utilization. To prevent traffic from queues that are forwarded first from starving traffic

from other queues, you can apply the WFQ queuing policy to set a minimum bandwidth for each queue.

•Weighted Fair Queuing

When you use WFQ, you divide the VC’s bandwidth and assign percentages to each queue (control, high, medium

and low). These percentages must be at least 10%, and must total 100%. This queueing policy is set on a per-port

basis.

•Weighted Fair Queuing with Strict Priority

With this combination of Strict Priority and WFQ, the control queue gets potentially all of the link bandwidth. The

remaining bandwidth is shared among the other priorities, according to the user-specified percentages. With this

policy, you specify percentages for the high, medium, and low queues only.

Note Currently, QoS only supports packets of Ethernet size 1514 and smaller. For

packets larger than this, the accuracy of QoS cannot be guaranteed.

QoS is triggered only when there is congestion on the link (i.e., the transmit queue for any VC reaches a pre-determined

depth). Until congestion occurs, the traffic is transmitted on a first-come-first-serve basis, and may not match the

requested percentages.