Re: [nsp] telco SCR/PCR and Cisco kbps

From: sthaug@nethelp.no
Date: Sat May 12 2001 - 02:45:19 EDT


> Not sure if its been changed in 12.0, but in 11.2, IOS output
> traffic shaping against ATM interfaces is configured with respect
> to 48 Byte Cell Payload.
>
> Therefore, with your PCR configured for 1536 kbps (kiloBITSpersecond):
>
> 1536 * 1000 = 1536000 bits per second
> 1536000 / 8 = 192000 bytes per second
> 192000 / 48 = 4000 cells per second

I'm afraid this is incorrect. The Cisco kbps value is converted to cells
using 53 bytes per cell, not 48. You can verify this for yourself by doing

debug atm events

and configuring a PVC. For instance:

interface ATM2/0.1 point-to-point
 pvc 0/110
  vbr-nrt 2304 1536 100

and you get as debug output the cell rates:

VBR: pcr 5433, scr 3622, mbs 100

1536 kbps gives you a cell rate of 3622 cps, not 4000.

It's this way in 11.1CC, 12.0S and 12.1 - I don't believe 11.2 is any
different.

Steinar Haug, Nethelp consulting, sthaug@nethelp.no



This archive was generated by hypermail 2b29 : Sun Aug 04 2002 - 04:12:37 EDT