Hello,
I have some doubts when thinking about LSA4/LSA5/LSA7 and Forward Address concept.
I know that in case of LSA5 the FA is set only in certain circumstances, but in case of LSA7 it is mandatory.
Assuming we are originating LSA5 with FA 0.0.0.0 (the NH for redistributed route does not recurse to
OSPF-enabled interface), the routers in OSPF domain have to route towards ASBR itself.
Inside the area where the ASBR reside it's fine, but routers outside this area don't know anything about ASBR,
so they need LSA4.
Now, let's say that we have met the conditions and we are originating LSA5 with FA set to IP address that is known
inside OSPF domain via LSA3.
In this case, routers in different areas other than the ASBR get to the ASBR or know how to get to the external route via the Forward Address,
and they should know how to get there by a LSA3, correct? The lookup is done via FA, not the Advertising Router.
In such case, is LSA4 completely useless? So why a regular LSA5 can't do this from the beginning?
Even if the NH of the external route is not an OSPF enabled IF, it can use another OSPF enabled interface,
and routers outside the area should know how to get to this Forwarding Address (but not to the Advertising router) via LSA3.
This way there wouldn't be the need for Type-4 LSA at all?
There must be some major flaw in my reasoning :)