Sforno on Deuteronomy 17:8/ספורנו על דברים י״ז:ח׳

1כי יפלא ממך, even though you have appointed competent judges in each town and city, in order that each court would dispense justice in its home base, if a doubt arises as to what the correct ruling is according to reliable tradition, the local court is not to hand down a ruling according to their opinion when it is not supported by authentic tradition, but the problem should be submitted to the Supreme Court, Sanhedrin. The same holds true when the judges in the local court were deadlocked.