  We report on a timing analysis of archival observations of the Anomalous
X-ray Pulsar 1RXSJ170849.0-400910 made with the RXTE Proportional Counter
Array. We detect a new large glitch (Delta nu / nu ~ 3 x 10^-6) which occurred
between 2001 March 27 and 2001 May 6, with an associated large increase in the
spin-down rate (Delta nudot/nudot ~ 0.3). The short time (1.5 yrs) elapsed from
the previously detected glitch and the large amplitude of the new spin-up place
this source among the most frequent glitchers, with large average glitch
amplitudes, similar to those of the Vela pulsar. The source shows different
recoveries after the glitches: in the first one it is well described by a long
term linear trend similar to those seen in Vela-like glitches; in the second
case the recovery is considerably faster and is better described by an
exponential plus a fractional change in the long-term spin-down rate of the
order of 1%. No recovery of the latter is detected but additional observations
are necessary to confirm this result. Observed glitch properties are compared
to those of radio pulsar glitches; current models are discussed in light of our
results. It appears that glitches may represent yet another peculiarity of
AXPs. Starquake-based models appear to be prefered on qualitative grounds.
Alternative models can be applied to individual glitches but fail in explaining
both. Thus the two events may as well arise from two different mechanisms.
