This is the Excess Burst Size, the extra amount of best-effort traffic beyond the committed burst the network will still attempt to carry, though it is the first to be discarded under congestion, and it defaults to the port's ifSpeed if not explicitly configured. An admin should check this value on any circuit where unexpectedly large excess bursts are being tolerated, since an unconfigured cktBe on a fast port could implicitly allow bursting up to full port speed. For example, leaving cktBe at its ifSpeed default on a T1 port means a customer's PVC could momentarily burst to nearly 1.544 Mbps of uncommitted traffic before hitting a hard limit.