Minimum you can send traffic upto 512kbps at a time. But when When you try to ping using large packets with prec 1 you can see some drops although the amount of traffic is way below 512k and the burst size is 12500 bytes.
Can you pls try to use other traffic generator like PAGENT which will have constant bit rate traffic? I think the ping traffic may be bursty and that's why when you tune the burst paramter, it helps to reduce the drops. Also, you can try to keep increasing the burst paramter until there are no drops.
