  We present the quasi-periodic slipping motion of flux rope structures prior
to the onset of an eruptive X-class flare on 2015 March 11, obtained by the
\emph{Interface Region Imaging Spectrograph} (\emph{IRIS}) and the \emph{Solar
Dynamics Observatory} (\emph{SDO}). The slipping motion occurred at the north
part of the flux rope and seemed to successively peel off the flux rope. The
speed of the slippage was 30$-$40 km s$^{-1}$, with an average period of
130$\pm$30 s. The Si {\sc iv} 1402.77 {\AA} line showed a redshift of 10$-$30
km s$^{-1}$ and a line width of 50$-$120 km s$^{-1}$ at the west legs of
slipping structures, indicative of reconnection downflow. The slipping motion
lasted about 40 min and the flux rope started to rise up slowly at the late
stage of the slippage. Then an X2.1 flare was initiated and the flux rope was
impulsively accelerated. One of the flare ribbons swept across a
negative-polarity sunspot and the penumbral segments of the sunspot decayed
rapidly after the flare. We studied the magnetic topology at the flaring region
and the results showed the existence of a twisted flux rope, together with
quasi-separatrix layers (QSLs) structures binding the flux rope. Our
observations imply that quasi-periodic slipping magnetic reconnection occurs
along the flux-rope-related QSLs in the preflare stage, which drives the later
eruption of the flux rope and the associated flare.
