Hi Magnus,
You do not need to set a CBS value, if you leave it off the command the switch will automatically choose a suitable value.
If you want to understand CBS then I will try to explain.
For any type of rate limiting (traffic shaping or policing) the switch (or router) measures the data rate over a time period.
This is because the device is not actually changing that actual transmission rate, it is just limiting the amount of data that the device is allowed to send.
For 250Mbps the switch makes sure the average data rate is limited to the specified rate. If the measurement time was 1 second then the switch would allow 250,000,000 bits to be sent, however we do not measure the amount of data in bits we measure it in bytes so that would be 31250000 bytes(250,000,000/8) in 1 second. This value is the CBS, basically the amount of data allowed to be transmitted in the time interval. In most cases it does not really matter what the CBS is, if we double the CBS it would double the time interval so we could send 62500000 in 2 seconds. The average is still the same but a larger CBS is better for dealing with large "bursts" of traffic. You need to be very careful about setting CBS on WAN links where for example a service provider may be rate limiting traffic received from a customer and the customer may be "shaping" traffic to the WAN. If the CBS values or "measurement intervals" are different you may find traffic being discarded.
In the example above if the customer outbound traffic is shaped with a CIR of 250000 and CBS of 62500000 but the service provider is policing incoming traffic to their network with a CIR of 250000 and CBS of 31250000. In this case the CIR is the same but because the customer CBS is 62500000 it is possible for the customer to send all 62500000 bytes within the first 1s of his 2s time interval, the service provider will however discard half of this because he is only allowing 31250000 in each 1s.
Hope this helps.
Regards Nigel
