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Frame-Relay Traffic Shaping


I have a question regarding the token bucket algorithm used for frame-relay traffic shaping.

What happens if a packet is larger than the token bucket?

For example a 1500 bytes (=12000 bits) packet with a Tc of 10ms, Bc of 5120 bits and Be of 0 bits.

Regards Robert


Re: Frame-Relay Traffic Shaping

For these Tc and Bc values CIR=Bc/Tc=512 Kbps. Your packet of 12000 bits will be transmitted in less than 3 Tc intervals (12000/Bc=2.34). Considering CIR=512K and the rate you want to transmit (12000 bbps) I think there'll be no problem. Check this:


New Member

Re: Frame-Relay Traffic Shaping

Hi Kivanc,

thanks for your mail. After reading your article on CCO I still have a problem of understanding.

The book "IP QoS" from Cisco Press says the following about traffic shaping on page 54:

"The maximum size of the token bucket is set to be the sum of conformed burst size, Bc and the extended burst size, Be. Tokens equivalent to Bc are added to the bucket every measuring interval T, where T = Bc/CIR. CIR is the allowed mean rate of traffic flow. If the bucket becomes full, any added tokens overflow. When a packet arrives, the token bucket is checked to see if enough tokens are available to send the packet. If enough tokens are available, the packet is marked compliant, and the tokens equivalent to the packet size are removed from the bucket. If enough tokens are not available, the packet is marked non-compliant and is queued for later transmission."

This means that a router can never transmit a packet larger than the bucket size.

But in reality, a router transmits a packet larger than the bucket size. It seems that for packets larger than the packet size, it can borrow bits from future intervals.

But I haven't found any document yet, which clearly describes this case.

Regards Robert

Re: Frame-Relay Traffic Shaping

To my understanding, if the packet size exceeds the token bucket size transmit of the packet will take more than one Tc interval, because shaper don't fragment packets.

Best Regards.

New Member

Re: Frame-Relay Traffic Shaping

You mean, the shaper can borrow bits from future intervals, if a packet has more bits than the bucket size?

Is there anybody at Cisco who can approve this?

Kind regards,


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