The code’s directory structure is trivial, FIB, mFIB, adj have their own directories.
for the most part, for all the FIB object types mentioned in this documentation there is a corresponding .h and .c file. As with any VPP component/sub-system a ‘public’ header file is any file that can be included by another sub-system and/or plugin. These must be specified in the build-system, so go look there. Public header files are always a good entry point to start reading.
There is no direct [VPP’s binary] API access to FIB, but FIB does expose types that can be used on the API by FIB and by other subsystems (e.g. Barnacles). These types are specified in fib.api and the encoding and decoding thereof in fib_api.[ch].
Most operations on a FIB entry happen as a result of an operation on a FIB table; an entry does not exist in isolation. The APIs in fib_table.h are well doxygen documented you should be able to figure out what they do. Use this as a starting point to explore how entries are created and deleted and how the source priority scheme works.
FIB sources are defined in fib_source.h. Each source behaviour has its own file fib_entry_src_*.c These define the virtual functions that determine how the source behaves when actions on the FIB occur. For example, what the entry must do when its covering prefix’s forwarding is updated.
When creating new paths/path-lists the main action required is to resolve them; see fib_path*_resolve, and once resolved to have them contribute a DPO for forwarding or for the uRPF list; see fib_*_contribute_forwarding and fib_*_contribute_urpf respectively.
The data-structures that used for entry lookup are protocol specific, they are implemented in separate files; ip4_fib.[ch], ip6_fib.[ch] and mpls_fib.[ch].
FIB extranet support is implemented in fib_attached_export.[ch]. FIB tracking is implemented in fib_entry_track.[ch]. FIB [back]walk is implemented in fib_walk.[ch].
Not much to say here, each adjacency type has it own file; use the force, read the source.
the majority of FIB coverage comes from the C Unit tests in fib_test.c. I strongly encourage you to add code here. It’s a much easier development cycle to fire up GDB, run VPP and iterate with ‘test fib’, than it is work in the python UT. You still need to write python UT, don’t get me wrong, it’s just easier to do the FIB dev using C UT.