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Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide
OL-14127-08
Chapter 7 Configuring the ATM SPAs
Configuration Tasks
Configuring Traffic Parameters for PVCs
After creating a PVC, you can also configure it for the type of traffic quality of service (QoS) class to
be used over the circuit:
• Constant Bit Rate (CBR)—Configures the CBR service class and specifies the average cell rate for
the PVC.
• Unspecified Bit Rate (UBR)—Configures the UBR service class and specifies the output peak rate
(PCR) for the PVC. This is the default configuration.
• Variable Bit Rate–Non-real Time (VBR-nrt)—Configures the VBR-nrt service class and specifies
the output PCR, output sustainable cell rate (SCR), and output maximum burst size (MBS) for the
PVC.
• Variable Bit Rate–Real Time (VBR-rt)—Configures the VBR-rt service class and the peak rate and
average rate burst for the PVC.
Each service class is assigned a different transmit priority, which the Cisco ASR 1000 Series Router uses
to determine which queued cell is chosen to be transmitted out of an interface during any particular cell
time slot. This ensures that real-time QoS classes have a higher likelihood of being transmitted during
periods of congestion.
Table 7-1 lists the ATM QoS classes and their default transmit priorities.
Ta b l e 7-1 ATM Classes of Service and Default Transmit Priorities
Service Category Transmit Priority
CBR 0 (highest)
VBR-rt 1
VBR-nrt 2
UBR 3
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