sub panel breaker box
For a subpanel application -- what you can do is pull the main breaker out of the panel and use a set of main lugs instead, as the panel disconnect is provided by the upstream feeder breaker (which will be a 30A unit given that your feeder is a 30A feeder).  In particular, since the 100A MCB BR panels use a backfed type BR main breaker, you can simply pull the main breaker out and replace it with a BRSF125 main lug kit.

Of course, if you want a single disconnecting means at the subpanel, you can leave the main breaker in there and use it as a disconnect switch.