What are the functions of the NULL0 route?
1. On the network with dynamic routes and default routes, if the convergence speeds are not synchronous, the temporary loop occurs.
2. Specify a NULL0 for the dynamic route.
A and B are dynamic routes. When B is not reachable, the packets are forwarded to the upstream routes. If the upstream routes do not aggregate, temporary loops occur between the links. Thus, to solve this problem, you need to specify an exact Null0 route to B, and the priority of the route is lower than that of the dynamic route.