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granting access to lower priority queues. The frame schedule provides each queue the bandwidth that
is allotted to it. This mapping is configurable both at the system and interface levels (as described later
in this section).
The four queues on any destination interface are configured to be part of the same CoS. Bandwidth is
not explicitly reserved for these four queues. Each of the queues 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.
The higher the WRR weight, the higher the effective bandwidth for that particular queue.
Tips
You can find the effective bandwidth (in Mbps) for a particular queue with the following formula:
(W/S) x B = n
Table 12 defines the values used in the bandwidth calculation formula.
Table 12
W
S
B
n
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 Catalyst 4000 Layer 3 Services
module software.
The Catalyst 4000 Layer 3 Services module software enables QoS-based forwarding by default. If it is
disabled, enter the following command to enable QoS forwarding:
# [no] qos switching
Use no version of this command to disable QoS switching on the entire system.
To configure QoS scheduling at the system level, perform the following tasks in global configuration
mode:
Task
Step 1
Set the mapping between IP precedence and the WRR
weight. See the command syntax description in Table 13.
Step 2
Return to privileged EXEC mode.
78-10164-03
Bandwidth Calculation Values
WRR weight of the specified queue
Sum of the weight of all active queues on the outgoing interface
Available bandwidth, in Mbps
Effective bandwidth, in Mbps
Installation and Configuration Note for the Catalyst 4000 Layer 3 Services Module
Configuring Layer 3 QoS
Command
Router (config) # qos mapping
precedence value wrr-weight weight
Router (config) # Ctrl-Z
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