after post process in booting sequence of router if no valid configuration file exists in NVRAM, the operating system searches for an available TFTP server.
my question is where this tftp server lies...and also want to know all the concepts regarding the use of tftp server in this booting process???
Loading an alternate boot image / new firmware or configuration settings. You should be able to disable it .
You provide the tftp server. It lies where you put it, but it has to be within range of broadcast it is going to be useful.
I have to assume that what the router wants to obtain from the tftp server varies from one router to another, so you don't expect a specific answer...
There are tftp server implementations for Unix, and it's so simple that there doubtless are for other OSes too.
The TFTP server should be in the router. You can use a TFTP client to upload new firmware images to the unit.
TFTP is used because it uses UDP which is simpler and had a smaller footprint than a full TCP-based FTP server.
TFTP is very typical in embedded devices that have limited RAM and ROM space.