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=9382&t=9268
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