  Stationary features are occasionally observed in AGN jets. A notable example
is the HST-1 knot in M87. Such features are commonly interpreted as
re-confinement shocks in hydrodynamic jets or focusing nozzles in Poynting
jets. In this paper we compute the structure and Lorentz factor of a highly
magnetized jet confined by external pressure having a profile that flattens
abruptly at some radius. We find the development of strong oscillations upon
transition from the steeper to the flatter pressure profile medium. Analytic
formula is derived for the location of the nodes of these oscillations. We
apply the model to the M87 jet and show that if the jet remains magnetically
dominated up to sub-kpc scales, then focusing is expected. The location of the
HST-1 knot can be reconciled with recent measurements of the pressure profile
around the Bondi radius if the jet luminosity satisfies $L_j\simeq10^{43}$
erg/s. However, we find that magnetic domination at the collimation break
implies a Lorentz factor in excess of $10^2$, atypical to FRI sources. A much
lower value of the asymptotic Lorentz factor would require substantial loading
close to the black hole. In that case HST-1 may be associated with a
collimation nozzle of a hydrodynamic flow.
