Cisco IOS traffic shaping allows traffic to burst at any time during the time interval.Within a time interval, the bit rate may be faster than the mean rate at any given time. In other words, Cisco IOS traffic shaping may transmit all committed bits in a burst at the start of the interval and then no bits for the remainder of the interval. Such scheduling can produce clumps of cells and cause the ATM switch to determine that the bit rate is violating the traffic contract. Refer URL

http://www.cisco.com/warp/public/121/paa1_shaping.html