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Cisco Policy Shaping Understanding Question

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
0
cpatte7372
Asked:
cpatte7372
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1 Solution
 
Jordan MedlenCommented:
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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cpatte7372Author Commented:
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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Jordan MedlenCommented:
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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cpatte7372Author Commented:
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
0
 
Jordan MedlenCommented:
No problem. Have a great day.
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cpatte7372Author Commented:
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
0
 
Jordan MedlenCommented:
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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cpatte7372Author Commented:
Cheers
0

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