  The forcing number of a perfect matching $M$ of a graph $G$ is the smallest
cardinality of subsets of $M$ that are contained in no other perfect matchings
of $G$. The forcing spectrum of $G$ is the collection of forcing numbers of all
perfect matchings of $G$. In this paper, we classify the perfect matchings of a
generalized Petersen graph $P(n,2)$ in two types, and show that the forcing
spectrum is the union of two integer intervals. For $n\ge 34$, it is
$\left[\lceil \frac { n }{ 12 } \rceil+1,\lceil \frac { n+3 }{ 7 } \rceil
+\delta (n)\right]\cup \left[\lceil \frac { n+2 }{ 6 } \rceil,\lceil \frac { n
}{ 4 } \rceil\right]$, where $\delta (n)=1$ if $n\equiv 3$ (mod 7), and $\delta
(n)=0$ otherwise.
