is it compulsary that in fajr salah we should (compulsary) do sunnath before faraz salah??

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The two Rak'as that are performed before the obligatory Fajr prayer are a recommended act and not an obligation. However, the Prophet never abandoned them even during travel. For more benefit in this regard, please refer to Fatwa 84719.

These two Rak'as should be performed before the Fajr obligatory prayer. However, if one was not able to perform them before the Fajr obligatory prayer, he may perform them after having performed the Fajr prayer or after sunrise, and the second is better as clarified in Fatwa 84847.

Allaah Knows best.