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This explains the difference between the Committed Burst Size (Bc) and Excess Burst Size (Be).
What is Bc allows traffic to burst to higher speeds, as available network bandwidth permits. A device can burst up to the Bc and still expect the data to get through.
Be describes the bandwidth available above the CIR up to the access rate of the link. Be is not negotiated, frames may be transmitted at this level but are most likely dropped?