The lwIP Port libraries (DspLwip.lib and DspLwipD.lib) are built with lwIP parameters that are suitable for most applications. Should application- specific tuning be required, this may be accomplished by modifying the lwipopts.h file, and recompiling the library.
A Code Composer Studio project, and all source code required, is provided with every MDK release. Find and open the lwip.prj file in the "$(MityDSP_2_1)\lib\lwip" directory (for MDK 2.1), modify the lwipopts.h file as required, and rebuild the debug and release versions of the library.
The lwipopts.h file is used to override the default options provided in lwIP's opt.h file. It is highly recommended that lwipopts.h is changed and not opt.h. Documentation on all available tuning parameters can be found in the opt.h file itself, or within the lwIP documentation at http://savannah.nongnu.org/projects/lwip.