Information regarding Bidah and Sunnah

Assalamou’alaikoum,
I was reading your book on the differences in the Ummat.
I have some confusion regarding the examples you gave between Bida’at and sunnat.
You said that it is only permissible to say ‘assalatou assalam a’laika ya Rasoulallah’ at thr grave of Nabi-e-kareem(SAW) only.
My question is, ‘Daroud Akbar’ is full of ‘assalatou assalam a’laika’ to the prophet(SAW) various titles. Is Daroud Akbar a bida’at then?