MIB: IF-MIB.
An estimate of the interface's current bandwidth in units of 1,000,000 bits per second. If this object reports a value of "n", then the speed of the interface is somewhere in the range from n-500,000 to n+499,999.
For the interfaces, which do not vary in bandwidth or for those where no accurate estimation can be made, this object should contain the nominal bandwidth.
For a sub-layer, which has no concept of bandwidth, this object should be zero.