FreeBSD: Burst

$fw add pipe 1 ip from 177.xx.xx.15 to any out via $ext_if
$fw add pipe 2 ip from any to 177.xx.xx.15 in via $ext_if
$fw pipe 1 config bw 128Kbit/s queue 16 burst 256K
$fw pipe 2 config bw 128Kbit/s queue 16 burst 256K

FONTE: http://www.fug.com.br/historico/html/freebsd/2011-10/msg00154.html