John,
I just tried this out, and the newer IOS versions (after 11.2) *will* let
you use a loopback interface as area 0 with different non-zero areas
defined on the spokes.
There is no reason for the traffic actually to travel over the area 0 link,
but area 0 must be in the hub router for the inter-area LSAs to be
advertised to the spoke routers. OSPF is just populating the IP routing
table; it is not making forwarding decisions. The router in this instance
will not try to send traffic over an extra link just because of an OSPF
rule about backbones. ;-)
Again, your mileage may vary, depending on IOS version.
Pamela
At 09:54 AM 6/21/01 -0400, you wrote:
>Yes, I'm replying to myself.
>
>While doing some reading it occurred to me why *not* extending area 0 across
>the WAN links should not work. In OSPF, unlike IS-IS, an area is defined by
>links, not routers. The rule states that interarea traffic must go through
>area 0. Well, if areas are defined by links, then this means that interarea
>traffic must at least go across one link that is defined as an area 0 link.
>
>In a hub-and-spoke environment with a single hub router, it seems to me that
>there just is no good way to use multiarea OSPF if you don't extend area 0
>across the WAN links.
>
>At least, that's the way it appears at the moment.
>
>John
>
>| I'm having trouble wrapping my brain around a specific scenario and I
>| wanted to get your thoughts. Let's say we have a hub and spoke network
>| with a single router as the hub. There are five areas attached to the
>| backbone. It seems that we would have to extend area 0 across the WAN
>| links, but I'm wondering what would happen if we didn't.
>|
>| If we didn't, the backbone router would have no interfaces in area 0.
>| I'm wondering if this would cause some major problems. I bet that it
>| would but I'm having a hard time thinking through what actual problems
>| might arise. Would this backbone router just "know" that it was area 0
>| because it has interfaces in multiple non-zero areas and hence behave
>| correctly?
>|
>| One obvious problem is that the backbone router would be a member of
>| every area and would thus be pretty busy if the network got to be very
>| big. If we extended area 0 across the WAN link the backbone router
>| would be protected from running SPF calculations everytime a remote area
>| had a link change.
>|
>| What other problems would arise? Would this even work at all? I don't
>| really have the tools to try it or I'd just attempt this chaos myself.
>| As you can guess, we run eigrp everywhere so I'm still clueless to some
>| of the workings of OSPF in a production environment.
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