为f0/0进入的流量，分配4M带宽送出到f0/1
-------f0/0--R1---f0/1-----
------> ------>
R1:
ip cef
class-map match-all bb2in
match input-interface f0/0
policy-map limit4m
class bb2in
police 4000000 750000 1500000 conform-action transmit exceed-action drop
int f0/1
service-policy output limit4m
经验公式：
BC=CIR/8*1.5
BE=2*BC
BC=4000000/8=500000*1.5=750000
