The C300 comprises most of the more important roads on the Isle of Bressay, opposite Lerwick. However, due to the relocation of the ferry terminal, this is no longer accessed by the C300! Starting at Heogan, towards the north end of the island, the C300 runs south along the Sound of Bressay to Maryfield, where a side road leads down to the ferry terminal. The route then continues to the harbour and main settlement on the island. It turns inland by the harbour and soon reaches a crossroads where all 4 roads are part of the C300. Continuing ahead, the C300 extends along Uphouse Road to Uphouse itself.
Going south from the crossroads, the route extends all the way down to the lighthouse at Kirkabister near the southern end of the island. in the other direction, the route extends north to the fork at Gunnista, and also includes the connecting road back to Maryfield.