Unanswered Question
Sep 24th, 2007


Need help on traffic shape question:

1) for a 5M traffic shaping on a FE port I have seen people use 5000000 as well as 5120000 shape rate. which is correct?

2) sometimes i see the interval of 125ms (TC) and some times I seen 100ms. does it matter?

3) if a service provider PE has a rate-limit command of 'rate-limit input access-group 101 5120000 250000 250000' does the CE also need to follow 'taffic-shape rate 5120000 2500000 2500000'.

4) what will happen if the CE is configured as 'taffic-shape rate 5120000 2560000 2560000'



I have this problem too.
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Paolo Bevilacqua Mon, 09/24/2007 - 11:16

1- little difference. but 5 x 10^6 is correct.

2- not really. But as higher the speed, the larger you want the interval.

3- Yes, if you find that more traffic than that leaves your router at any time.

4- Probably nothing appreciable by users, just more or less buffer for burst.

Hope this helps, please rate post if it does!

Joseph W. Doherty Mon, 09/24/2007 - 18:06

2) Depends what you're trying to accomplish. Adjusting the TC impacts how the bursty the shaper can be as it releases packets to the wire.

3) Good idea, otherwise if your traffic exceeds the PE's setting, your excess will be discarded on the PE.

4) The CE will try to throttle data to more or less simulate the overall transfer rate of a physical circuit of 5.12 Mbps.

san-pandey Mon, 09/24/2007 - 22:11

thanks guys.

now question on traffic shape with voice traffic. I read somewhere on Cisco website that for critical traffic the TC should be 10ms. The below sample is for 2M traffic shape on 10M full duplex Ethernet connectivity.

shape average 2000000 20000 0

does this configuration look ok for 2M shaped traffic with bc that will allow tc=10ms?



san-pandey Tue, 09/25/2007 - 03:28

2- not really. But as higher the speed, the larger you want the interval.

what is the rational for this? so does that mean that higher bandwidth should have larger interval, but if you have voice you should have 10ms?

for the question on my previous post I have got the answer. i just tried it on a router and looked at sho traffic-shape. the command puts the tc at 10ms.

Interface Fa0

Access Target Byte Sustain Excess Interval Increment Adapt

VC List Rate Limit bits/int bits/int (ms) (bytes) Active

- 2000000 2500 20000 0 10 2500 -



Joseph W. Doherty Tue, 09/25/2007 - 03:38

I believe you're correct about the need to reduce the TC so critical traffic such as voice doesn't have to wait as long for the next interval. Think it helps reduce jitter.

I further believe your on the correct track in your example for obtaining a 10 ms TC. One of the show traffic-shape commands will show whether you've set the TC as you desire. (I often used 10 ms TCs even for just transactional traffic, but don't recall the number of zeros to drop from the CIR parameter.)

I also recall there's often a minimum TC (8 ms?) and IOS will complain if you attempt to go beyond the minimum.


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