Calculate vbr-nrt on a ATM interface?

Question

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

Solution

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