Re: OSPF Area

[Brian Candler <B.Candler at pobox.com>]

On Mon, May 20, 2002 at 08:51:22AM -0700, Barry Raveendran Greene wrote:
> If you wait until you have 50 to 100 routers in your network before you
> design hierarchy into your network, then be ready for pain. Lots of pain.

What I meant was, if you divide your network into multiple OSPF areas when
you have four routers in one POP, it's a waste of time. You are unlikely to
know for sure what your topology will look like in one or two years time,
and therefore you're unlikely to build the right routing hierarchy.

If you want to plan for the future, then you try to aggregate. That is, try
to keep all your leased-line customers within the same netblocks. Keep your
modem pools in a separate IP block (they are easy to renumber anyway).

That's the theory. In practice, leased-line customers are hard to renumber,
and when you end up moving a leased line from one router to another router,
or from one POP to another, your aggregation will break anyway. At that
point, you may be better off carrying your customer routes in iBGP, and
inject nothing into OSPF except loopbacks.

Aggregation is not the be-all and end-all anyway. If your IGP ends up
carrying a few hundred routes there should be no problem.

My own rule of thumb for scaling: if you are building a national-scale
network with POPs in multiple towns, then put your backbone routers in OSPF
area 0 (or ISIS level 1) and the POPs themselves in their own OSPF areas (or
ISIS level 2)

If you are building a single POP with a handful of routers, you are unlikely
to get any advantage from multiple areas. Concentrate on building your POP
with robust links (e.g. 100M or gigabit ethernet point-to-point) which don't
flap.

B.

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