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The Airspeed Unreliable procedure is not necessarily the first thing you do when you get an IAS Disagree alert.
The condition for the procedure is: Airspeed or Mach indications are suspected to be unreliable.
The objective of the procedure is: To identify a reliable airspeed indication, if possible, or to use the Flight With Unreliable Airspeed table in the Performance Inflight chapter for the remainder of the flight.
According Boeing items which might indicate unreliable airspeed are:
•Difference between captain and first officer airspeed indications
•Speed/altitude information not consistent with pitch attitude and thrust setting
•Blank or fluctuating airspeed indication
•Continuous or intermittent stick shaker
•IAS DISAGREE alert
•ALT DISAGREE alert
•AOA DISAGREE alert
•SPD failure flag
•SPD LIM failure flag
•Erroneous minimum speed bars
•Erroneous maximum speed bars
•Airspeed low aural
•Airspeed low PFD indications on one side
•Overspeed warning
•Erroneous Flight Director (FD) pitch command bar
•Radome damage or loss
If the AOA DISAGREE alert is shown, one or more of the following flight deck effects can occur:
•Erroneous Pitch Limit Indicator (PLI)
•Windshear alerts
•Autoslat operation