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We have a DS3 ATM with a 3Mb (soon to be 6Mb) PVC. How do you calculate the vbr-nrt setting on a Cisco router for this interface?

Understanding the VBR-nrt Service Category and Traffic Shaping for ATM VCs

interface atm1/0.1
pvc 1/100
  vbr-nrt <Peak Cell Rate (PCR) in Kbps> <Sustained Cell Rate(SRC) in Kbps> <Maximum Burst Size(MBS) in Cells>

<1-149760>  Peak Cell Rate(PCR) in Kbps
<1-1>  Sustainable Cell Rate(SCR) in Kbps
<1-65535>  Maximum Burst Size(MBS) in Cells
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up vote 1 down vote accepted

This is the rule I use here (and working fine on several routers), using an empirically based method:

  • PCR = MBS = 95% of access speed
  • SCR = 1

So in your example it will be fine with this:

interface atm1/0.1
 pvc 1/100
  vbr-nrt 2850 2850 1

Where 3000Kbps * 95% = 2850Kbps

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