Normal Burst Setting for Rate-Limiting (CAR)

[michaeljmorris]

I have a several DS-3s with PVCs on them that I rate-limit to 9Mbps. My original rate-limiting config is :

interface Hssi4/0.600 point-to-point

rate-limit output 9000000 5625000 5625000 conform-action transmit exceed-action drop

This set a Normal Burst of 5,625,000 bytes (or 45,000,000 bps = DS-3).

However, I've been reading on CAR and how it differs from Traffic Shaping, particularily this page :

http://www.cisco.com/warp/public/105/policevsshape.html#tokenrefreshrate

This says the token refresh rate for CAR is a constant refresh based on a formula that uses the CIR not the Normal Burst. The Normal Burst actually determines the initial size of the Bucket, but does not determine how many tokens are added back to the bucket over time (that's what the CIR is for).

So, my question is, what is a good value for the Normal Burst? I've done some work in Excel and it seems the Normal Burst just makes it longer until CAR drops a packet.

Thoughts and suggestions ?

Thanks.

Michael J. Morris

[geoff.belknap]

Picking a normal burst size really depends on the type of traffic on the circuit.

My rule of thumb is

- Lots of bursty traffic / large MTU?

Increase Normal Burst Size.

- Intermittent bursts / small MTU?

Decrease Normal Burst Size.

Here is a good bit of information along the same lines:

http://www.cisco.com/en/US/customer/tech/tk543/tk545/technologies_tech_note09186a008010dd6a.shtml

As an aside to your question, I notice you've specified 5625000 as your 'burst-normal' and 5625000 as a 'burst-max'. The 'burst-max' is only required if you specify 'violate-action' (which would add a second bucket to the token algorithm).

-Geoff

[michaeljmorris]

I found this page :

http://www.cisco.com/en/US/tech/tk543/tk545/technologies_q_and_a_item09186a00800cdfab.shtml#policing

Which says the formula for setting Bc with CAR is :

CIR [BPS] *

(1 byte)/(8 bits) *

1.5 seconds

Based on this, my Bc would be 1,687,500 (9,000,000 * (1/8) * 1.5). But, why is 1.5 seconds chosen as the "typical round trip time"?

Also, if I leave the Bc at 5,625,000 an I creating a situation where CAR would not kick in? I've done some Excel calcuations that appear to support that. This would be bad since I pay a telco bill based on BW, and going over 9Mbps could cause large charges.

Thoughts ?

Mike