If you have a number of routers on the end of each WAN
link then you might want to extend area 0 to include
those links and terminate on the remote routers. You
might also be able to get away with not really having
an area 0 on an actual network by creating a loopback
interface and placing it by itself in area 0. Im just
shooting that last one off the top of my head and have
never tried it but it might work. Also dont forget
that static routing works very well for a hub and
spoke topology; this is if you don't have other
redundant connections and you can redistribute between
static and eigrp. To me it sounds like your hub and
spoke wan links are your backbone. I would probably
try and extend area 0 over those wan links. It also
depends on your remote routers. I've got a 1600 on
the end of a 56k line running tcp header compression
that stays around 8%-10% processor usage, if youv'e
got smaller routers like that and are pushing a lot of
traffic through them and doing things like compression
or encryption you might want to look at the processor
utilization pretty closely. I've found OSPF to be a
pretty low bandwidth user so I wouldn't worry about
WAN link congestion if only the hub and spoke are in
area 0 (of course if you extend the other areas over
those WAN links and there are numerous recalculations
in that area your network is in effect sending useless
data over the WAN link and also possibly affecting all
the other areas by slowing the hub router down). In
closing, after babbling for quite a while, my gut
would say to go with area 0 over the WAN links. blah
blah, blah blah blah blah blah. Then again I could be
wrong :)
Ben, CCNP
--- John Neiberger
wrote:
> 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.
>
> Regards,
> John
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