A lot of books recommend a small but high-speed LAN at the hub site as the
area 0. This may not apply in your case, but might help you think of other
options.
Caveat: This may be a recommendation that comes from Cisco who wants to
sell more routers!? ;-)
Priscilla
At 12:52 PM 6/21/01, John Neiberger wrote:
>In this scenario, with a small number of routers an area zero isn't
>really necessary. However, expand this to 100 routers in 20 or so
>groups and an area zero starts to make sense. However, in my original
>post I wasn't concerned with best practices. I was only curious as to
>how OSPF would behave if I were to try to configure it that way.
>
>I was trying to reconcile a couple of things. I've always read that in
>multiarea OSPF, all interarea traffic must go through area zero. If
>areas are defined by links and not routers, then does that mean all
>traffic must flow over a link defined as area zero? That was really the
>issue and Pamela answered that for me.
>
>This scenario occurred to me while reading the area zero rule and then
>thinking about a hub-and-spoke configuration with a single hub router.
>I would never suggest that someone actually configure the network that
>way, I simply was wondering how OSPF would behave in a hub and spoke
>network with no area zero configured. As it turns out, the loopback
>interface can be placed in area zero, thus fulfilling that requirement.
>
>John
>
> >>> "Chuck Larrieu" 6/21/01 10:28:12 AM >>>
>A couple of questions / thoughts
>
>In the scenario mentioned, is an area zero really necessary? I.e. why
>not
>throw all routers into a singe area, whatever it's name? In hub and
>spoke,
>all inter-spoke traffic will have to go through the hub anyway, no
>matter
>what the protocol.
>
>Another thing to keep in mind, is that OSPF does not in and of itself
>change
>the way routing works. When a router receives a packet, it checks the
>destination address, compares this to routes in the routing table, and
>if
>there is a match, forwards the packet out the appropriate interface. It
>does
>not say "hmmm, I have a directly connected interface that matches, but
>this
>is an OSPF router, therefore I will forward the packet to the backbone
>router first" :->
>
>How's stuff, Pamela?
>
>Chuck
>
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf
>Of
>Pamela Forsyth
>Sent: Thursday, June 21, 2001 9:09 AM
>To: [EMAIL PROTECTED]
>Subject: Re: OSPF Hub and Spoke [7:9268]
>
>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.
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
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