Hi Chuck,
I feel like I have grasped some of OSPF, and understood without any doubt in
my mind all but two of your comments.(It's possibly down to all but one
after writing this post)
The first one was: Do OSPF area numbers have to be unique. I've re-written
this about 4 times while I tried to think it out and read through my books.
What sort of flow of LSA's are there between the two discontiguous area's
with identical area numbers. LSA type 1& 2 flooded between the two? or are
they just treated as separate areas. I'm thinking the latter but changing my
mind every second.
The second was: Do virtual links have to be point to point?
What I probably want here is the argument against my logic, or confirmation
of it.
Using:
Area_3-----area_2----area_1------area_0
Area 2 has a virtual link to area 0. When I did this I noticed that the ABR
between area two and one had all of it's interfaces in area 0, as well as
the 'actual' area of each interface. (So a router with 3 interfaces had for
instance, 3 interfaces in area 0, 2 interfaces in area 2 and 1 interface in
area 1)
Area 3 has a virtual link into the router above. As the router above *is
now* within area 0, the virtual link ends there, and the router in area 3
now has a virtual link to Area 0 (And therefore now becomes part of the
backbone area 0)
So my answer would be, yes, virtual links have to be point to point. The
first virtual link extends area 0. The second virtual link merely connects
to the extended area 0.
I need to press send quickly before I change it all again.
Thanks for your time,
Gaz
""Chuck Larrieu"" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> Agreed that there are few if any written materials that do a decent job of
> explaining how OSPF works and why. Disclaimer - I have not read Tom
Thomas'
> book. But I have read a number of others, including the RFC. Plus my Lab
> prep work.
>
> One problem is that most of us don't understand the difference between
> routing and routing protocols. Routing protocols are one means, but only
> means, of getting routes into the routing table. Routing forwards or drops
> packets based on the contents of the routing table.
>
> Lack of understanding of how and why OSPF operates can lead to question
like
> "when there is a virtual link connecting a discontiguous area to the
> backbone, does traffic travel across the transit area to the backbone, and
> then back into the transit area?"
>
> Area_2----area_1------area_0
>
> If there is a virtual link from area 2 into the backbone, does traffic
bound
> for area 1 from area two have to go to area 0 first? The answer is two
fold.
> The area 2/area abr is now part of area 0. This allows area 2 to learn
about
> area 1 routes. And secondly, when a router receives a packet, it is the
> routing process ( not the routing protocols ) that determine the
forwarding.
>
> Another question - do OSPF area numbers have to be unique? Answer is no
they
> don't. in a multiple area network, as long as there is an area 0, all
other
> areas can have the same number. there is nothing in an LSA that identifies
a
> route with an area. A corollary of this is that in a single area network,
> there does not have to be an area 0. Why?
>
> Got a good one. Do virtual links have to be point to point?
>
> Area_3-----area_2----area_1------area_0
>
> You can create a virtual link from area 3 to area 2 and a second link from
> area 2 to area 0 - it works. Good example of this in Slattery's book.
>
> Over time I have come to appreciate the Cert zone / Berkowitz approach,
> which is to attain understanding of how things work. one might be
surprised
> at the number of people with very high level certification who can make,
but
> who don't know why it works.
>
> Chuck
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
John
> Neiberger
> Sent: Friday, June 22, 2001 7:32 AM
> To: [EMAIL PROTECTED]
> Subject: RE: OSPF Hub and Spoke [7:9268]
>
> Yep, you are correct. I see now that as long as non-zero areas receive
> LSAs from a router configured as area zero, then the routing tables will
> be built and all should be well. This makes more sense to me, anyway,
> but some of what I've read made the waters murkier rather than clearer.
>
> How many times have you read "All interarea traffic must go through
> area zero" ? I've read that in several places. It's almost true, but
> not if you want to split hairs, especially since OSPF defines areas as
> links, not routers. It should read that for loop-free routing to take
> place, all non-zero areas must connect to area zero only. This is more
> correct and doesn't imply that interarea traffic must cross an area zero
> link.
>
> Does that sounds about right? :-)
>
> >>> "Chuck Larrieu" 6/21/01 11:15:31 PM >>>
> John, this one's got me to thinking a little bit. Your kinda right but
> kinda
> wrong.
>
> The areas are an OSPF structure, used for the building of the SPF
> tables.
> It's not that inter area traffic has to go through a discreet area 0,
> but
> that in OSPF in order for an area to learn about routes to another
> area
> there has to be an area 0 router in between them. It does not matter
> if
> there are a number of interfaces that are ABR's, or if there is a
> discrete
> and pure area 0.
>
> With OSPF, all that matters is that the appropriate adjacencies are
> formed,
> and that the LSA's are processed and that the OSPF database is created.
> If
> all that occurs, OSPF routes will be placed into the routing tables. As
> far
> as the router itself is concerned, routing is independent of the
> routing
> protocols involved.
>
> I've fooled with this in the past. I'll have to do another Q&D lab to
> gather
> some evidence, and post it here over the weekend.
>
> In the meantime, for those interested in some in-depth discussion of
> routing, Howard's white paper on Certification Zone is definitely
> worth
> reading. I have not seen the likes of it in any other source,
> including
> Doyle ( although it has been too long since I've read Doyle )
>
> Chuck
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf
> Of John
> Neiberger
> Sent: Thursday, June 21, 2001 6:55 AM
> To: [EMAIL PROTECTED]
> Subject: Re: OSPF Hub and Spoke [7:9268]
>
> 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.
> |
> | Regards,
> | John
> |
> |
> |
> |
> _______________________________________________________
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