Im fairly sure I understand most of the rules regarding area zero, but have
heard a few people referring to "the area zero rule". Is there a fixed
definition of the area zero rule somewhere?
Thanks,
Gaz
""John Neiberger"" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> 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.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=9421&t=9268
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