Hi Kalyan,
Please go through below links...
http://scn.sap.com/docs/DOC-8042
764393 Configuration of the SAP System Landscape Directory
SLD export/import FAQ note: #1540435.
You can also maintain two SLDs, in portal and other one in PI. And step-up a bridge between Portal and PI SLD so that you can have high availability of SLD keeping both in sync.
Regards,
Prithviraj.