I am working with a Juniper router which has the capacity to hold 60M routes in the RIB and 4M routes in the FIB. Should the router crash when more than 4M routes are being sent to FIB, considering no policies have been defined to limit the routes that can be pushed into FIB?
Should the routes crash when more than 4M routes is being sent to FIB, considering no policies have been defined to limit the routes FIB can hold?
If your concern is related to hardware limitation, then the first 4 million FIB entries will be installed and anything after that will not. You should also be receiving quite a lot of alarms indicating you are starting to exhaust your FIB memory.
In cases where you exceed the memory threshold, related processes are going to crash, core dump, and attempt to recover. This concept is not specific to Juniper, any BSD/*nix based system will react this way when a process runs out of memory.
If your concern is related to a licensing restriction, Juniper does not enforce FIB scale licenses in software. FIB entries will be installed until you reach your respective hardware's limitation, at which point the behavior will align with #1.
If you need more detail, feel free to add a comment and I can update my answer.