I used that phrase to refer to the fact that non-backbone OSPF areas
must use the backbone to communicate; they cannot communicate directly
to each other. To save myself some typing--after saying the same thing
several times--I used shorthand and referred to this as the area zero
rule. There's probably another term for this aspect of OSPF routing but
if there is, it's slipping my mind at the moment.
>>> "Gareth Hinton" 6/21/01 2:53:35 PM
>>>
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:
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