OSPF multi-level hierarchy: Necessary at all?

avg@kotovnik.COM (Vadim Antonov) writes:

Well, actually it is not that bad. The biggest number of locations is
probably found in AT&T phone network - 250 or so. Sprint is in few
dozen. The existing IGPs are quite happy with that kind of complexity,
so if you belong to the "one-router-per-POP" school of thought the
IGP complexity is a non-issue.

Well, the phone system is already hierarchial. The top-level is pretty
small, it is the second and third levels which are monsters.

There are about 100,000 NXX's in the country. California has the largest
state with about 12,000. The Los Angeles LATA is the biggest at about 5,000.
Within PacBell there are about 800 CLLI locations for the Los Angeles LATA,
not including all the other CLECs who may have locations in LA. Getting from
the relatively few IXC access tandems to those 800 locations is the trick.

I may be doing something wrong, but I've found OSPF gets a bit cranky
with far less than 800 routers and 5,000 routes in an area.

Hmm. THis is the right direction for this discussion - if someone built
telephone network over IP technology (even if it's not public internet),
he need quite scalable IGP protocol.

Through you dont' need plain IGP schema - you have backbone (with 100 -
300 nodes) and regional access networks - every not too large. It can be
build just as any ISP+customers. It's not so beautiful as multi-level
scheme, but well designed and work just fine (OSPF+IBGP backbone,
OSPF+iBGP customers).