Managing Baselines in Primavera P6 Professional is a slightly different process than you would encounter in file based tools such as Microsoft Project. When you save a baseline in Microsoft Project, you are saving copies of dates, durations, resource assignments and costs to special fields within the project file. However, when you create a baseline in Primavera P6 Professional, you are actually creating a complete copy of the project, and then linking back to that project in order to display baseline values.
This article takes you through the differences and the process http://ow.ly/qYlTx [1]