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22-4
ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter 22 Configuring Quality of Service
IP Precedence Based Class of Service (CoS)
About Scheduling and Weighted Round-Robin
Frame scheduling becomes increasingly important when an outgoing interface is congested. To handle
this situation, network administrators can assign weights to each of the different queues. This provides
bandwidth to higher priority applications (using IP precedence), while also granting fair access to lower
priority queues. The frame schedule affords each queue the bandwidth allotted to it by the network
administrator. This mapping is configurable both at the system and interface levels (as described later in
this chapter).
The four queues between any pair of interfaces are configured to be part of the same service class.
Bandwidth is not explicitly reserved for these four queues. Each of them is assigned a different
WRR-scheduling weight, which determines the way they share the interface bandwidth. The WRR
weight is user configurable; you can assign a different WRR weight for each queue.
Tip The higher the WRR weight, the higher the effective bandwidth for that particular queue.
You can find the effective bandwidth (in Mbps) for a particular queue with the following formula:
(W/S) x B = n Mbps,
where
W = WRR weight of the specified queue
S = sum of the weight of all active queues on the outgoing interface
B = available bandwidth in Mbps
n = effective bandwidth in Mbps
For example, if W is 4, S is 15, and B is 100, the formula would be (4/15) x 100 = 26 Mbps, and the
effective bandwidth for the specified queue in this example is 26 Mbps.
Configuring Precedence to WRR Scheduling
This section describes the Cisco IOS commands necessary to configure QoS mapping at the system and
interface levels. The commands described in this section are unique to the Layer 3 switching software.
Layer 3 switching software enables QoS-based forwarding by default.
To configure QoS scheduling at the system level, use the following command:
To set the precedence back to the default setting for the switch router, use the no version of the qos
mapping precedence command.
Table 22-2 shows the default WRR weights for IP precedence.
Command Purpose
Router(config)# qos mapping precedence value
wrr-weight weight
Sets the mapping between IP precedence and the
WRR weight.