Qos Shape WAN Bc doubt

Unanswered Question
Sep 22nd, 2010


I have one router configured qos shape as below.Shape-wan is applied at branch side as well as hub side link, link is point to point.

As per the below configuration, Bc is 640000 & link bandwidth is 30Mb.

Is the Bc value correct and 30Mb BW will be availble for the tarffic over the WAN?

policy-map shape-wan
  class class-default
   shape average 30720000 640000

interface GigabitEthernet0/1
service-policy output shape-wan

I have this problem too.
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Peter Paluch Fri, 09/24/2010 - 14:40


Yes, the configuration is correct, the shaping will be performed at the level of 30 Mbps. Regarding the Bc value, it does not influence the average bandwidth but rather the size of the data burst. There is no special default and if you decided to use the size of 640 kbit then fine, you should just observe whether your applications are running fine with this setting. The relation of CIR, Bc and Tc (Tc is the time in which tokens are refilled to allow further data to be sent when shaping) is given by the formula

CIR = Bc / Tc

Modifying the Bc with the CIR being constant results in different Tc values being used, which in turn translate into different queueing latencies for your packets when being deferred by the shaping mechanism to the next Tc interval waiting for necessary tokens.

Best regards,



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