In general the input error occurs due to runts, giants, no buffer, CRC, frame, overrun, and ignored counts. Other input-related errors can also cause the input error count to be increased, and some datagrams may have more than one error; therefore, this sum may not balance with the sum of enumerated input error counts.
-- check your memory size and buffer for input queue.
-- check the CPU utilization, because it cannot able to handle the more no. of data comes input to the interface.
-- check the load or traffic given to that 2611 device.