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=9374&t=9268
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