Hi there, I'm studying for CCNP routing (BSCI) at the moment and I've read and re-read OSPF many times. I think I've pretty much got most of it sorted but one major area which still eludes me is that of OSPF areas.
From what I gather the point with the various area types
not so stubby
NSSA totally stubby area
is to try and have the minimum required amount of LSAs in each area (and therefore reduced number of routes, processing, etc.).
Ok this seems fine to me but in all of the documentation I've read so far (including Cisco course notes), none of them actually give much information about these areas apart from which types of LSAs are allowed in.
Ok so on that basis why not just create every area that doesn't have an ASBR as a totally stubby area thus reducing LSAs, routes, etc? It seems to me that every area that's not area 0 only needs a default route out of the area into area 0, then area 0 would know how to reach all the other areas. Obviously it can't be as simple as this or there wouldn't be all the area types but I just can't seem to find any decent documentation that gives a simple explanation. I'm sure it's not difficult - I just need to find some documentation or have someone explain to me.
Please can anyone point me to any OSPF documentation that gives a good explanation of exactly what each area is and what is and why you'd need more than just a default route?