  We review some of the basic features and predictions of a gauge invariant
spontaneous Lorentz symmetry breaking model arising from the nonzero vacuum
expectation value of the electromagnetic tensor and leading to a nonlinear
electrodynamics. The model is stable in the small Lorentz invariance violation
approximation. The speed of light is independent of the frequency and one of
the propagating modes is highly anisotropic. The bound (Delta c)/c < 10^{-32}
is obtained for such anisotropy measured in perpendicular directions.
