I am using enterprise library logging block 5.0. My solution has over 20 projects, most of them are class libraries. When configuring the enterprise library, do I need to add app.config in all the projects and create logging related sections in each of the config file?
No, you don't have to duplicate logging settings in all projects. You can create it once, store it in a single file or database and then use non-default configuration sources to share as described in Scenarios 2 and 3 of Developer's Guide. Appendix D - Enterprise Library Configuration Scenarios