+/**
+ * \brief Get or create a bridge domain.
+ *
+ * Get or create a bridge domain with the given bridge domain ID.
+ *
+ * \param bdm bd_main pointer.
+ * \param bd_id The bridge domain ID or ~0 if an arbitrary unused bridge domain should be used.
+ * \return The bridge domain index in \c l2input_main->l2_bridge_domain_t vector.
+ */