I have a customer with 10 plants, who runs MRP in an external system. This system reads all planning data from SAP, makes its own calculations (It uses forecasting procedure) and it produces Purchase Requisitions who then feeds them into SAP for the planners to review them and convert them to Purchase Orders. This customer of mine asked to make the exercise of using MRP, and more specifically Forecasting MRP of SAP, in the place of the external system.
One option was to choose some materials in some plants (ie the same 10 materials in 3 out of the 10 plants), we make all proper customization and run them through SAP MRP directly, so we see how it goes and then fine-tune them. This suggestion was rejected in the sense of while fine-tuning we could run out of stock. Next suggestion was to run SAP MRP alongside and at the same time with the external system MRP. This is something i object too, since (a) SAP MRP will "see" the PRs external system will produce, and will not produce correct results in its own run; we would have to erase all external system's PR and run SAP MRP so we have a "clear" picture.
My last, and more logical suggestion for me, is to create a long term scenario where i capture all these materials and these plants within a scenario. Then, i will be able to -anytime- check what happens to these materials, and also run comparisons between LTP scenario and real time data with the external-driven MRP. The only concern i have in this option is that in LTP i can not make changes to Safety Stock, as this is transmitted to the real planning data as well.
Please advice, since i need to find the "correct" way to move forward t this exercise.