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22-12
ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter 22 Configuring Quality of Service
IP QoS—Functional Differences Between Modules (Catalyst 8540 MSR)
Note The IP QoS feature is only applicable for enhanced Gigabit Ethernet and enhanced ATM Router Modules
installed in the Catalyst 8540 MSR chassis.
Input Policy
All functionality, such as classification, marking, metering, and policing, is the same for both the
enhanced Gigabit Ethernet (XPIF based) interface module and the enhanced ATM Router Module.
The difference is that all incoming traffic to the enhanced Gigabit Ethernet (XPIF based) interface
module received on the cable is treated as ingress traffic that is eligible for input policy functions.
On the enhanced ATM Router Module, there is no physical connectivity, so traffic that comes in from an
ATM interface to the enhanced ATM Router Module is eligible for input policy functions. This traffic
stream can egress an Ethernet interface, or an enhanced ATM Router Module interface (and egress
through an ATM interface). However, the traffic stream coming from an Ethernet interface and egressing
an ATM interface is not eligible for input policy functions on the enhanced ATM Router Module.
Output Policy
The functionality for queue selector and congestion management is the same for both the enhanced
Gigabit Ethernet (XPIF based) interface module and the enhanced ATM Router Module.
The difference is bandwidth allocation. On the enhanced ATM Router Module, bandwidth allocation is
calculated using the following scheduler class weight formula:
WRR(A) = 255 * (Bandwidth of A) /[(Total Bandwidth for IPQoS config) + 500,000 K]
This formula is used because the enhanced ATM Router Module handles traffic from both Ethernet and
ATM interfaces, where 500,000 KB of bandwidth is always reserved for ATM traffic. This bandwidth is
only used for scheduler class weight calculation. The unused bandwidth can be used by ATM or Ethernet
traffic because of the weighted round-robin (WRR) scheduler.
Differentiated Services for ATM Forum VCs
The differentiated services for ATM forum VCs enables the enhanced ATM Router Module to treat ATM
traffic with better granularity, providing minimum assurance for a particular traffic class when the
enhanced ATM Router Module is operating at congestion level and beyond.
Since rate scheduler is not available on the enhanced ATM Router Module, in the earlier implementation,
eight scheduler classes of one WRR scheduler were used, as shown in Figure 22-4.