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Interface Bandwidth Statement vs Parent Shaping Policy

Hello everyone - I have been looking at QoS on a WAN router and wanted to throw this topic out for discussion...

I have a WAN router (3945) that is connected to a MPLS network with an ethernet handoff.  The physical connection to the router is on G0/1 and is connected at 100/Full.  The connection speed provided by the ISP is 50M.  I have a QoS policy already defined that I need to apply which accounts for voice, video, critical apps, etc. main question is - is there any difference in the following two methods of applying that QoS policy:

1.  Set bandwidth statement on G0/1 to 100M and then apply a parent shaping policy to 50M with a child policy calling out my QoS policy.


2.  Set bandwidth statement on G0/1 to 50M and then just apply my QoS policy


Is there any difference?  Is one of these considered a best-practice approach?


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Re: Interface Bandwidth Statement vs Parent Shaping Policy


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Any difference?  Yes, #2's QoS policy will only engage when your port congests at 100 Mbps.  #1 is the correct approach.


With a 3945, your IOS will probably shape with a 4 ms Tc rather than 25 ms, but if it does use 25 ms, suggest reducing it to 10 ms if you're going to pass VoIP.

Some shapers (I believe) don't allow for L2 overhead, i.e. they're only measuring the L3 rate.  You're provider, though, is very likely to enforce 50 Mbps at L2.  If your shaper doesn't account for L2, you'll need to shape 5 to 15% slower to allow for L2.  This insures your provider doesn't drop over-rate packets, which can be critical to avoid when supporting apps like VoIP.

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