OSPF multi-level hierarchy: Necessary at all?

Btw. Flat network...

Routers now have - 128 or 256 MB RAM, 300 - 400 Mhz CPU. I guess you can
built flat betwork with 5,000 routers withouth hard problems.

Mmm...I wouldn't be so sure, actually. Recall we need to refresh each LSA every
30 minutes not to let some bugs drive our network crazy. Also the more routers
you have in the flat network the greater is the size of LSDB (or whatever table
is used) and, since all routers see all other routers and links, the more
CPU time
we need to spent calculating our SPFs, maintaining the RT, etc...

Through it's not the question. There is _already_ 2 levels; you can use
multi-zone OSPF (independent OSPF networks connected by _redistribute_).
The question was _is 2-level hierarchy enougph__?

Externals in OSPF can also become a scalability issue---Type5 LSAs have
domain-wide flooding scope....so, unless you want your areas to be stub, you
have all your externals crossing all areas and installed into LSDBs of all
making them spend more time on SPF, while it is understood that the lower
the level of hierarchy a router is in, the less granularity of external
routing info
it needs.