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ATM Switch Router Software Configuration Guide
OL-7396-01
Chapter25 Configuring ATM Router Module Interfaces
Configuring VC Bundling with IP and ATM QoS
The “active” scheduler-classes concept is very important. A scheduler-class is said to be “active” if there
is traffic on that class. If there is no traffic on that class, then the bandwidth reserved for that class is
used by other classes when sending traffic. So, the formula to calculate the bandw idth can be modified
as follows:
In this formula, notice that the bandwidth reserved for the four IP QoS classes (1, 6, 7, and 8) is
approximately half of what is actually configured in the class-map (for example, voice traffic gets 90
when actually 200 is configured). This is because the available enhanced ATM router module bandwidth
for IP QoS is considered to be 500 Mbps, not 1Gbps. Th is is because on the enhanced ATM router
module ATM traffic must also be handled.
Another important concept is that the bandwidth reserved for a particular class, for example voice,
(90Mbps in this case), is for all XPIF interfaces configured to send traffic to this enhanced ATM router
module. Traffic from all XPIF interfaces is queued in this way on the enhanced ATM router module.
Finally, excessive traffic on a particular queue can hog the bandwidth if it has a high scheduler-class
weight. For example, if the requirement for voice is only 1.2 Mb ps, but it has been configured such that
the scheduler weight allows 90 Mbps, that much voice traffic could be sent.
This explanation describes traffic coming from Ethernet and ATM interfaces into the enhanced ATM
router module. When traffic leaves the enhanced ATM router module and is transmitted out of the OC-3
interface, all ATM guarantees are preserved by the switch fabric. For example, if traffic enters from the
Ethernet interface and exits from OC-3 through the enhanced ATM router module, then there are two
phases to this process. Phase 1, Ethernet-to-WRR and then, phase 2, the enhanced ATM router module
sents the traffic as rate scheduled and WRR-to-OC-3.
So, if traffic exits from the enhanced ATM router module on a CBR PVC to the OC-3 interface, it is rate
scheduled (which is similar to Strict Priority). The same is true for the SCR portion of the VBR traffic.
The remaining traffic, such as UBR, is WRR sche duled as usual.
So, if only a 1.2Mbps VC is available for voice, then only that much should be sent from the Ethernet
interface. If more traffic is sent, it will reach the enhanced ATM router module but, from the enhanced
ATM router module to the OC-3 interface, the traffic is dropped due to the rate scheduling mechanism.
Table25-1 Scheduler Class to Weight Calculation
Scheduler
Class
Number Traffic Type
Scheduler-
class
Weight
Bandwidth on
Enhanced ATM Router
Module (Mbps)
1 Default IP traffic 16 28
2 CBR 240 423
3 VBR (RT and nRT) 128 225
4 LSIPC 255 —
5 UBR, and traffic from Ethernet ports
that do not support IP QoS
64 113
6 Priority IP traffic 25 44
7 Video 44 77
8 Voice 51 90

Schedule-class weight of Scheduler-class-A

(Bandwidth of scheduler class A) = * 1Gbps

of all “active” scheduler-class weights

Σ