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question about cbr & ubr configuration


I encountered two types of atm configuration as follows:

1. atm pvc 1 1 51 aal5snap

2. pvc ABC 2/51

ubr 1000

encapsulation aal5snap

Is config 1 applied QoS like cbr or ubr ? Is it no max bandwidth limited at this config ?

Some one said that config 2 was cbr . Is it true ? (I don't think so) If not, does Cisco support cbr and how to configure? Does ubr 1000 mean the max bandwidth of this pvc is 1M?

Thanks a lot.



Re: question about cbr & ubr configuration

1.Atm pvc interface configuration command(atm pvc vcd vpi vci [aal5snap | ilmi | qsaal] [peak_rate average_rate [burst_size]] [oam seconds]) is used to create a PVC on the Catalyst 5000 series switch interface.

The atm pvc command creates a PVC and attaches it to the VPI and VCI specified. You cannot specify both vpi and vci as 0.

If you are configuring an SVC, you must use this command to configure the PVC that handles the SVC call setup and termination. In this case, specify the qsaal keyword.

The peak_rate argument determines the size of the rate queue used. ATM PVC traffic-shaping software release 50.1(1) and later, and software release 51.1(1) and later dynamically create rate queues to satisfy the specifications of atm pvc commands. When an atm pvc command specifies a peak rate that does not match any use-configured rate queue, a rate queue is created dynamically.

In your example , the configuration is atm pvc 1 1 51 aal5snap. In this peak_rate average_rate and oam parameters are omitted(optional). If the peak_rate and average_rate options are omitted, the PVC defaults to the highest bandwidth rate queue available. By default, the VC is configured to run as fast as possible.


2.The pvc command replaces the atm pvc command, which, although still supported and available, will become obsolete in the near future.The pvc command creates a PVC and attaches it to the VPI and VCI specified. The Cisco IOS software dynamically creates rate queues as necessary to satisfy the requests of the pvc commands.

Refer document :

The UBR configuration option allows you to define the Peak Cell Rate (PCR). There is no burst size that will limit the number of cells transmitted at the PCR. Coupled with the coarse granularity of PCR, it causes the Unspecified Bit Rate (UBR) to continuously send data above PCR.

Refer the folowing document to find the calculation of PCR:

Hope this information is enough.

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