QoS shaping peak / average

Unanswered Question
Mar 22nd, 2008

Hi,

I am working on QoS exam for CCIP and I have the following question:

What is the difference between:

shape peak 64000

and

shape average 128000 8000 0

In both cases, the traffic rate will be about 128kbps right ?

Thank you !

I have this problem too.
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Joseph W. Doherty Sat, 03/22/2008 - 06:14

My understanding is, they shouldn't be the same, except for the first time interval.

From: http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/reg_pkt_flow_shaping.html#wp1046537

"When the Be size equals 0, the interface sends no more than the burst size every interval, achieving an average rate no higher than the mean rate. However, when the Be size is greater than 0, the interface can send as many as Bc+Be bits in a burst, if in a previous time period the maximum amount was not transmitted. Whenever less than the burst size is transmitted during an interval, the remaining number of bits, up to the Excess Burst size, can be used to transmit more than the burst size in a later interval. "

Edison Ortiz Sat, 03/22/2008 - 08:58

In both cases, the traffic rate will be about 128kbps right ?

Yes, but the average and interval isn't the same.

Here is the output from the first command:

Match: any

Traffic Shaping

Target/Average Byte Sustain Excess Interval Increment

Rate Limit bits/int bits/int (ms) (bytes)

128000/64000 2000 8000 8000 125 2000

Adapt Queue Packets Bytes Packets Bytes Shaping

Active Depth Delayed Delayed Active

- 0 0 0 0 0 no

Here is the output from the second command:

Traffic Shaping

Target/Average Byte Sustain Excess Interval Increment

Rate Limit bits/int bits/int (ms) (bytes)

128000/128000 1000 8000 0 62 1000

Adapt Queue Packets Bytes Packets Bytes Shaping

Active Depth Delayed Delayed Active

- 0 0 0 0 0 no

HTH,

__

Edison.

Joseph W. Doherty Sat, 03/22/2008 - 16:33

Edison, by answering "yes", you're saying that both commands will provide a sustained 128 Kbps average bit rate?

Edison Ortiz Sat, 03/22/2008 - 17:29

I didn't say yes to that because that's not what the OP asked.

Traffic rate != Average rate

See the output that I posted.

Joseph W. Doherty Sat, 03/22/2008 - 17:42

Yes I did look at your post. Further confused by "Traffic rate != Average rate"

The OP question was "In both cases, the traffic rate will be about 128kbps right ?", and your answer started with "Yes, but the average and interval isn't the same. ". So I thought your "yes" implied both shape commands would deliver 128 Kbps, even with the addition of the differences in Tc.

My point was the two commands should deliver different sustained traffic rates, which is what I thought the OP might be asking. Again, not clear, at least to me, what point you're trying to make.

Edison Ortiz Sat, 03/22/2008 - 18:53

Again, not clear, at least to me, what point you're trying to make.

Is it because I contradicted your post?

Again, as per the output I posted and I copied the syntax verbatim per OP's post, the traffic target rate from both commands is the same and I believe that's what the OP was getting to. The Shaping Average Rate provides a different value and I'm not arguing both commands do produce different results.

The first command will only achieve 128kbps during 'excess' burst conditions while the second command will achieve 128kbps under normal conditions.

I hope this is clear enough as I'm not going to repeat the same thing once again.

Joseph W. Doherty Sat, 03/22/2008 - 20:44

"Is it because I contradicted your post?"

Well, when I read your original post, I didn't see the point you were making, and the "yes" did seem a contradiction.

Only reading your last reply, where you explicitly mention how the two commands show the same "traffic target rate" do I now understand the "average" you refer to in your original post also refers to what the two commands display as the average rate.

Ditto for "Traffic rate != Average rate". Again, only now I understand you're referring to command parameter displays, not delivered rates.

You believe Benoit's question was how the two shape commands display their parameters. I thought Benoit may have thought they both deliver the same rate (i.e. Bc + Be = 2x Bc; or peak 64k = avg 128k).

So, there isn't any contradiction, we were both answering what we thought Benoit was asking.

"I hope this is clear enough as I'm not going to repeat the same thing once again."

Yes, it is, and thank you!

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