Traffic Shaping - Confusion

[enderkaran]

Hello Everyone,

I am a beginner on QoS and little bit confused so far, i saw an example such below:

For example, to achieve a CIR (contract rate) of 128Kbps, the Cisco router will send 16000 bits (Bc = 128000 * 0.125) during the first interval and stop sending any further traffic until the next interval. So, if the access link speed is 1.544Mbps, it will take the Cisco router 10.3ms to send 16000 bits. For the rest of the interval, it will not send any traffic.

The point that i am confused :  It divides 16000bits to interface physical speed (1.544Mbps). but basicly, shaping is to limit our traffic right ? so either we have 1.5 Mbps or 1gig interface, above example forces user to 128Kbps access speed. Then why it devides amount of data that is sent in each interval to access speed ?

it is something like that I am connected to my wifi modem with 54Mbps but my contract is 8Mbps with ISP. for any speed measurement, i should consider 8Mbps which ISP gives me. So in the example above, does access speed matter or CIR rate configured on shaping is our max speed ?

[Peter Paluch · Accepted solution]

Hello Ender,

An interface always works at its nominal speed, so for example, regardless of the policing or shaping configured, a 1Gbps interface will always send frames at the link speed of 1Gbps. The only way to slow down data transfers is to interesperse their sending with artificially inserted silent intervals. That will make the data transfer speed to approach the configured rate but your have to average the amount of sent data over longer periods of time. That is also the reason why it is necessary to divide the shaping or policing rate by the true link speed to know how long will a transmission of this amount of data take on that particular interface. Your router may send data at the link speed for at most this amount of time, and for the remainder of a single second, it must postpone sending of the data.

Please feel welcome to ask further!

Best regards,

Peter

[enderkaran]

Hello Peter,

Thanks for your reply. I see the point.

I will keep posting on forum for my further questions. Appricate for it.

[Joseph W. Doherty]

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Posting

Peter has explained how a shaper works.  Additionally, though, if you have a contracted bandwidth, the ISP should be able to tell you the particulars for how they are measuring your 8 Mbps.  Specifically, you'll want to know what time interval(s) they are measuring against.  You'll then want your shaper to measure against the same time interval(s), or less, so as to avoid sending a burst that the ISP will consider out-of-contract (and which the ISP will likely drop "excess").

PS:

Oh, and to further clarify a few points.  All actual data transmissions don't have to be at the beginning of the measured time period, it can be "randomly" dispersed.  However, if there's pending/queued/buffered traffic, it will stop and start as Peter describes.

Measured time periods are generally subsecond.  Older Cisco IOS generally used 25 ms; newer 4 ms.  (NB: as you reduce the time interval, a shaper more closely emulates an actual interface for that rate.  The downside with using reduced time intervals, it increases the shaper's processing overhead.)