default TC value of policy on cisco isr router

[Herman2018]

Hi, anyone can please help advise the default TC value of qos policy on cisco isr router4331? Thanks in advance. 

[pieterh]

TC value is not set, but computed!
source: Compare Traffic Policy and Traffic Shape to Limit Bandwidth - Cisco

Shaping increments the token bucket at timed intervals that use a bits per second (bps) value. A shaper uses the next formula:

Tc = Bc/CIR (in seconds)

In this equation, Bc represents the committed burst, and CIR stands for committed information rate. (See Configuring Frame Relay Traffic Shaping for more information.) The value of Tc defines the time interval during which you send the Bc bits in order to maintain the average rate of the CIR in seconds.

The range for Tc is between 10 ms and 125 ms. With distributed traffic shaping (DTS) on the Cisco 7500 series, the minimum Tc is 4 ms. The router internally calculates this value based on the CIR and Bc values. If Bc/CIR is less than 125 ms, it uses the Tc calculated from that equation. If Bc/CIR is more than or equal to 125 ms, it uses an internal Tc value if Cisco IOS^® determines that traffic flow can be more stable with a smaller interval. Use the show traffic-shape command to determine whether your router uses an internal value for Tc or the value that you configured at the command-line. The next sample output of the show traffic-shape command is explained in show Commands for Frame Relay Traffic Shaping.

[Herman2018]

thanks all for your responses and advice. we didn't set BC, only issue the command "shape average 100000000 " , and BC value is 400000 when running show policy-map. Based on the formula, the TC is 4ms. What is the best practice for TC value or BC value for 100M circuit? there are different types of traffics , voice , video, network control .... . Thanks. 

[Joseph W. Doherty]

I'm a bit surprised Cisco is defaulting to the minimum (?) value of 4ms.

I don't recall a best practice for a Tc or Bc value.

Personally, again, I've usually found 10ms suitable for most situations, regardless of port's actual physical bandwidth or for traffic type.

A lower Tc more closely mimics a physical port for the same bandwidth, for its short term average bandwidth consumption, but does, at noted by @Elliot Dierksen, increases processing overhead (but as ISRs have gotten progressively "faster", perhaps that's why Tc defaults have decreased).

BTW, just out of curiosity, I configured a 100 Mbps shaper (shape average 100000000), in PT, using its "4331" and see:

Router#sh policy-map interface 
 GigabitEthernet0/0/0

  Service-policy output: test

    Class-map: class-default (match-any)
      0 packets, 0 bytes
      5 minute offered rate 0 bps, drop rate 0 bps
      Match: any
      Traffic Shaping
           Target/Average   Byte   Sustain   Excess    Interval  Increment
             Rate           Limit  bits/int  bits/int  (ms)      (bytes)
        100000000/100000000 977516 3910065   3910065   39        488758

        Adapt  Queue     Packets   Bytes     Packets   Bytes     Shaping
        Active Depth                         Delayed   Delayed   Active
        -      0         0         0         0         0         no

Interestingly, it shows, in above, an "Interval" of 39ms.

[Elliot Dierksen]

I think this is still the recommendation for voice, but that was a TC of 10ms. I rarely see frame relay these days, so I don't keep up on that. The lower the TC, the closer your traffic will stay to the limits you configure. It does increase the CPU load a little bit on the router as TC gets smaller, but I doubt that is a cause for concern in a 4K.

[Joseph W. Doherty]

Over the years the default Tc has changed (later default Tc being a smaller value).

Don't know for sure what's the current default, which can change per IOS version.

If you want to know default, enter police and/or shape command without optional parameters and "see" what's actually being used.

If you don't like default or you want to guarantee a particular Tc, use optional Bc/Be parameters to set Tc.  (NB: I don't know but perhaps in later IOS versions Tc might be set as a parameter. {I recall some later IOS variants support buffer setting using ms.})

BTW, I recall (?) years ago default Tc was often 25 ms later it was 10 (?) ms.  Also BTW, 10 ms is often good in many cases (e.g. shaping VoIP).