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shape average values in a policy map. megabits to bps conversion

I have a WAN link that is 10 meg and we are responsible for shaping it so packets are not lost end to end.


For 10 meg if I do shape average 10000000 this seems to give me good consistant performance around 9.6 mbps using JPerf between sites.

But this value is bits per second, and if the site link is really 10mbps, well isn't the conversion between 10 megabits per second equal to 10485760 bits per second?


If I put shape average 10485760 Jperf comes down a bit around 7 - 8 Mbits/sec so perhaps the telco is dropping packets.


I just wanted to understand the math involved in this shape command.


Cisco 2811 12.4(25g) ADVIPSERVICESK9-M



  • WAN Routing and Switching
Super Bronze

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I think 10 Mbps Ethernet is 10,000,000 bps, not 10,485,760.  However, if you're dealing with a logical bandwidth limit, it depends on what value your provider is using.  Also, when enforcing a bandwidth limit, measurement interval become important, and those too are up to how your provider is configured.

Additionally, 10 Mbps is the raw transmission speed of the Ethernet.  It doesn't include L2 or L3 overhead.  So, again, it's up to your provider to determine what their 10 Mbps bandwidth limit means (they should be able to tell you what/how they are measuring).

Years ago, I worked with a Tier 1 provider, whose logical limits attempted to simulate Ethernet wire rate for that "bandwidth".  I.e. if you had a 10 Mbps cap, useful bandwidth should be like original 10 Mbps Ethernet.

On Cisco routers (including 2811s) I would usually shape about 15% slower, as I believed most Cisco shapers don't account for L2 overhead.  (The 15% is about L2 overhead for average packet sizes.  Overhead percentage, though, varies with frame size.)