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Cisco Policy Shaping Understanding Question

Posted on 2014-04-24
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Last Modified: 2014-05-13
Hello Experts,

BT have provided us with a 70Mb circuit. They have configured the following policy on their router:

!
policy-map parent_shaper
 class parent
  shape average 64400000 257664 0
!
!

Can someone take a look at the policy and let me know in layman terms what BT are trying to achieve?


Cheers

Carlton
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Question by:cpatte7372
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Expert Comment

by:Jordan Medlen
ID: 40019924
This is basically saying that they are rate limiting the connection to ~64Mbps committed information rate with a committed burst size of ~258kbps and 0bps excessive burst size. This is how they're achieving the "70Mbps" service that they are providing you.
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Author Comment

by:cpatte7372
ID: 40019951
Jordan

Thanks for responding. However, I still don't understand how that will give us our 70Mbps. The burst won't take us to 70Mb.

Regards
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Accepted Solution

by:
Jordan Medlen earned 500 total points
ID: 40019974
It will get you close. It will shape all of the traffic flowing through the configured interface, which isn't posted here, to that committed information rate.

You could call and ask them to bump it to ensure that you're getting what you're paying for.
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Author Comment

by:cpatte7372
ID: 40020036
Jordan,

Cheers mate.

I'm going to put a call into BT and find out what's going on....

I will wait to see if they're any more comments before allocating points


Cheers
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Expert Comment

by:Jordan Medlen
ID: 40020268
No problem. Have a great day.
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Author Comment

by:cpatte7372
ID: 40028069
Hi Jordan,

I questioned BT as you suggested and I got the following response:

You are correct in that there is a mismatch between how CDR is sold and how the Cisco and other vendors implement QoS on CPE, also the accuracy of the shaper/scheduling feature can differ with the device whether software or ASIC based.
 
BT put all CPE through rigorous testing to understand how the hardware and operating system maps to various rates of CDR and EF/AF rates within it. The resulting configurations match the CPE hardware and Operating system QoS function against the platform CDR model.
 
CDR is not quite an IP rate but includes a degree of L2 header. The issue is you have a subtle difference between the two. The CPE therefore needs to shape to 32.5 to allow for the inter-packet gap, differences in frame sizes and therefore %overheads. This is the optimum value to get the most throughout from the CDR ordered.”


Can I get your opinion on the response?

Cheers

Carlton
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LVL 6

Expert Comment

by:Jordan Medlen
ID: 40028141
It does make sense, shaping isn't an exact science. I can see that depending on a multilayer switch vs a router, how it can differ. As long as you are getting close to the speeds that you are supposed to get, I wouldn't bother with it so much. If you are drastically off, say a several or more Mbps, then that's a problem.
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Author Closing Comment

by:cpatte7372
ID: 40063070
Cheers
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