Questions

It is any routing protocol used within an autonomous system.

It is a host that has a multiple network interfaces and that requires multiple IP addresses is called as a Multi-homed Host

A typedef defines a new name for a type, and in simpler cases like the one shown below you can define a new structure type and a typedef for it at the same time.

typedef struct

{

    char name[20];

    int age;

} emp;

However, in the structure defined in this question, there is an error because a typedef declaration cannot be used until it is defined. In the given code fragment the typedef declaration is not yet defined at he point where the link field is declared.