Hi Stef I have one flow, I mark as EF class, then I use CBQ to control rate in tc script there is place to control queue size (buffer size) for example ## Definition of the CBQ leaf classes to support EF and BE ## EF class $TC class add dev eth1 parent 2:1 classid 2:5 cbq bandwidth 100Mbit rate 6Mbit avpkt 1000 prio 1 bounded allot 1514 weight 600kbit maxburst 10 defmap 0 $TC qdisc add dev eth1 parent 2:5 pfifo limit 10 ..... here use pfifo inside EF class, the parameter "limit" is queue size 10 However, each NIC card has txqueuelen which is transimission queue of device, the default is 100 So I am confused here, if I want control EF class buffer size, I think I should change "tc ......limit ...." but what about NIC queue size? does NIC queue size is only for BE traffic? thanks yuxiao