Is the Sunnah prayer of Fajr to be prayed after the Fajr Adhan, or after the obligatory Fajr prayer?

If you are unable to perform the two Rak'ahs of the Fajr Sunnah prayer before the congregational prayer begins, you may perform them after the congregational prayer has concluded, or after sunrise until just before Dhuhr. The time for Fajr begins with the true dawn. To ascertain its legal time, one should refer to the scholars of the land.