11. Given real numbers $x_{1}, x_{2}, x_{3}$ satisfy $x_{1}^{2}+x_{2}^{2}+x_{3}^{2}+x_{1} x_{2}+x_{2} x_{3}=2$. Then the maximum value of $\left|x_{2}\right|$ is $\qquad$.
11. 2 .

From the condition, we have
$$
x_{1}^{2}+\left(x_{1}+x_{2}\right)^{2}+\left(x_{2}+x_{3}\right)^{2}+x_{3}^{2}=4 \text {. }
$$

Notice that,
$$
\begin{array}{l}
x_{1}^{2}+\left(x_{1}+x_{2}\right)^{2} \geqslant \frac{x_{2}^{2}}{2}, \\
x_{3}^{2}+\left(x_{2}+x_{3}\right)^{2} \geqslant \frac{x_{2}^{2}}{2} .
\end{array}
$$

Therefore, $x_{2}^{2} \leqslant 4$, which means $\left|x_{2}\right| \leqslant 2$.
When $x_{1}=x_{3}=-1, x_{2}=2$, we have $\left|x_{2}\right|_{\text {max }}=2$.