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SCR and PVC question

Can you possibly answer a hotly debated issue involving one of our ATM WAN circuits

We have a 7206 VXR with a 34Mbs ATM interface

The service provider is proving a VBR+ 2Mbs PVC with 4Mbs Peak Cell Rate

I?ve configured the router interface as follows

interface ATM1/0.2 point-to-point

bandwidth 34000 ! I accept this is incorrect

ip address x.x.x.x

pvc BRIR02 2/50

vbr-nrt 4000 2000 50 ! my colleagues tell me this ought to be 4000 4000 50

inarp 10 ! but I think this is correct

oam-pvc manage

oam retry 3 3 3

encapsulation aal5snap



I?m willing to admit I?m wrong but I?d like a second opinion first

If we have a 34Mbs bearer, 2 Mbs Sustained Cell Rate with 4Mbs Peak Cell Rate then why would I need to change the config to 4000 4000 50?

Regards Tony


Re: SCR and PVC question

When you configure VBR, your SCR is the average rate you'll be transmitting at, while the PCR is the peak rate you'll send at for very short periods of time when you have lots of data to send. The size of that burst depends on your MBS value, or 50 in your case. Your credit to burst increments when you transmit at less than SCR for a period of time, but it will never go over 50 cells.

With what you've said, if the provider sold you a 2Mb SCR circuit with a 4Mb burst, you've configured it correctly. Your buddies are trying to tell you to make PCR and SCR equal at 4Mb, which will make your average rate 4Mbps, and eliminate any burst (if you're running newer code, it wont let you configure PCR=SCR with a burst value, since it is meaningless).

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