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traffic shaping burst value

pfrancis3
Level 1
Level 1

Hello, I have a Cisco 881 router connected to a 10Mbps WAN. I am trying to set up traffic shaping. I am trying to establish the correct burst value to use within the command 'shape average x x'.
In order to do this, I understand I need to establish the correct time interval. I found on Google that " Most Cisco routers have a Tc (time interval) default value of 125 ms" therefore I should set my shape average command to be 'shape average 10000000 1250000'. Is my logic correct, or should I be taking other factors in to account ?
Thank you for any advice.

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Joseph W. Doherty
Hall of Fame
Hall of Fame

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Hmm, I recall (?) Cisco used to use a default Tc of 25ms, and I also recall (?) newer IOS implementations changed the default to 4ms (this, I suspect, to better support VoIP).

Setting Bc to obtain an optimal Tc, depends on what your service needs are.  If you're shaping to deal with a downstream bottleneck, and your doing QoS, traffic with tight timing requirements, generally a smaller Bc (and Tc) is required.  This because, as you minimize Bc (and Tc), the shaper performs closer to a physical link of similar bandwidth.  However, I believe this adds to processing load, so you want to avoid needlessly doing this.  Personally, I either just go with the default or, if I set Bc, I've found a 10ms Tc usually works well.

BTW, when working with a rate limiter (or policer), as they don't queue over rate traffic, using a small Bc (and Tc) tends to drop (bursty) traffic.

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