[nsp] CAR ?

From: Kevin Gannon (kevin@gannons.net)
Date: Thu Jun 28 2001 - 11:48:31 EDT


I am looking at applying the follwoing CAR config:

interface ATM2/0.1 point-to-point
 description *****Connects To interface ATM5/0/0.11 On 7507 Bunker B*****
 bandwidth 18000
 ip address 19.46.189.2 255.255.255.252
 rate-limit output access-group 102 13000000 9000 9000
       \conform-action transmit exceed-action drop

What my questions are:

1. This will allow a sustained rate of 13Mb/s ?
2. I will be able to busrt a Max of 9000Bytes (72000Bits)
   over the 13Mb/s. In practice will this give me a max
   of a tiny bit over 13Mb/s. ?

3. How does the tokening system work if the user is only
   running at 2Mb/s for an hour and suddenly bursts
   to 40Mb/s do the tokens get kept for the hour ?

4. I note that Cisco have a formula:

   normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds
   extended burst = 2 * normal burst
 
   Normal Burst= 13,000,000/ 8 * 1.5 = 2437500
   Extended = 2437500 * 2 = 4875000

   They recon for TCP traffic on average you will only achieve the
   configured rate (in my example 13Mb/s). Based on the figures
   above this seems like the customer could burst to 19.5Mb/s .

5. How do you calculate for how long and how often you can burst.
   In my example can I burst to 19.5Mb/s for 1.5s every minute
   every 1.5s*2 ??
   
6. Is there anything I should be weary of I dont really want the
   customer bursting at all but I do not want a bad config to
   affect there CIR ?

7. Is there any papers on the finer details of CAR ?

Thanks & Regards,
Kevin



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