How to see and set transmit interval (Tc) in QoS

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Aug 13th, 2009
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Anybody knows how to see the transmit interval (Tc)? I tried the command "show traffic-shape", but looks like that command only works for frame relay traffic shaping. I configured "police" in ethernet interface, with bandwidth of 320000 bps without specifying the Bc. The router then automatically set the Bc to 1/32 of the CIR, which is 10000 byte. If we follows the CIR=Bc/Tc, then Tc=Bc/CIR, then we got the Tc = 0.25 s. The problem is, from what I've read from cisco documentation, Tc is between 10 ms - 125 ms. This means that the value from the router doesn't conform with the document. How could this happen? I tried this using cisco 7200 series.

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Gregory Camp Thu, 08/13/2009 - 15:40
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I think you are confusing traffic shaping with policing. The policer adds more tokens to the bucket based on the formula below. Tc has no meaning in policing.

[(Current_arrival_time - previous_arrival_time) * CIR]/8

If you want to configure shaping, use the "shape average" command. Once you configure this you can run "show policy-map interface" and see the Bc value. From this you can calculate Tc.

On the ISR platform you can pretty much set the Tc to whatever you want. I personally would leave it at the default value for Tc = 4ms in 12.4 code.

prima.ramadhan Thu, 08/13/2009 - 18:25
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Hmm.. okay. Anyway, how to set the Tc in ISR platform? I haven't found the way yet.

Gregory Camp Fri, 08/14/2009 - 11:06
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There is no way to directly set the Tc. You need to configure Bc and the CIR to make the Tc you want using the formula below.

Tc = CIR/Bc

policy-map shape-out

class class-default

shape average 320000 Bc_VALUE

Joseph W. Doherty Fri, 08/14/2009 - 03:46
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If you're using the traffic-shape command, the 2nd and 3rd parameters configure Bc and Be, which will adjust Tc.


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