Once working with OSPF, you will have a case that one of the routers is connected to the internet – so that’s the ASBR.
We will need that all our routers, in our OSPF network in area 0 and any other area, to be able to go to the internet. For this, we need to inform them to go via the ASBR because he is the router who has the internet. You got lost? No problem, let me show it to you via an illustration.
You can see, R2 is the ASBR because he has one interface connected to the Internet. This router has internet but our goal is to allow all routers in the whole OSPF network to also go to the internet (in area 0 and in area 1).
That means we want that all routers (except the ASBR) to have a default route pointing to the ASBR as being their gateway.
We could create static default route on each of the routers ourself, but think if you have 70 OSPF routers then you have to create 70 static default routes and that’s a lot of work to do. Instead, we can run one command on the ASBR and all routers participating into the OSPF will receive that default route.
To make it easy, let me run a LAB to show you how this can be done.