Riverstone Networks WICT1-12 - Manual (Page 142)

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.
8.4.1 Configuring QoS (Multi-Rate Line Card)
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.