how is road from bangalore to bandipur national park and which route is the best to get there ?

Bandipur National Park is
about 100 Kms from Mysore.

On the Bangalore – Mysore – Ooty Road.
This is the easiest road route.

Mysore – Nanjangud (45 Kms)
Nanjangud – Gundulpet (25 Kms)
Gundulpet – Bandipur (18 Kms)

There is only one train to Nanjangud from Mysore in the morning.

The potholes from Gundulpet to Bandipur is occassionaly is strewn with road.

It can be bad for your car. Buses generally do the stretch anyways. Since it is on the way to Ooty.