Re: OSPF areas

From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Wed May 21 2008 - 10:41:18 ART

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Hi Yemi,

When you area using a single ospf process on each router, in order to
have disconnected area 0s, they would have to be defined on different
routers. Each router is making its normal route selection decisions
based on the routes is receives, and they do not have an additional link
state database.

The problem with disconnected area 0s is not that OSPF gets confused,
just that another rule starts taking effect if you also have active
neighbors in multiple areas - you begin ignoring summary LSAs learned
(from another ABR) via a non-backbone area.

Connect three routers using three non backbone areas so that the routers
form a single triangle. Then add a loopback interface on each router
into area 0. You will still have full connectivity even though there
are three disconnected area 0s. Then add a new router to any of the
three existing area 0s...

Paul.

Salau, Yemi wrote:
> No it's not, for the so called discontiguous networks, you can use
> tunnel interfaces/vpn to bridge into the backbone area, be careful
> though on the real environment.
>
> You don't want to have 2 Area0's do you, at least not what the RFC
> recommends if you don't want to confuse the hell out of your OSPF
> routers ... ie. which link-database to believe. So, you ideally also
> want to "bridge" the two Area0s. What of if you're merging two different
> companies' OSPF networks, well ... that's where BGP comes in handy, I'm
> sure there are other ways to achieve this also.
>
> One thing I need to also note is that it's possible to have multiple
> instances of a non-backbone area as long as they have ABR connecting to
> the Backbone area, and you don't need a virtual link in this case.
> Although, the Design/Architect guys would normally object to this, as
> it's suboptimal and can cause issues. Still looking/digging through the
> RFC that mitigates against the fundamental design principles for this
> though.
>
> So if you have Area2-------Area0------Area2 This needs no virtual
> link/tunnel interfaces, and will work, I've done it several times in my
> office; but is not best practise from design perspective
>
> But if you have Area0------Area1-------Area2 You have to hook Area2 up
> back to Area0 (hope this make some sense); Golden OSPF Area Rules: All
> areas must have a link/leg in Area0, and there should only be one Area0
>
> Many Thanks
>
> Yemi Salau
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> ahmed badr
> Sent: Wednesday, May 21, 2008 12:53 PM
> To: Ina&Laurean
> Cc: Miguel Trejo; Cisco certification
> Subject: Re: OSPF areas
>
> In cisco CCIE Exam certification guide, the author recommends that if
> you
> have discontiguous or partitioned area, you should link it by Virtual
> link.
> this is to have the same LSA database in the area.
> I LABed it without virtual links and it worked. *is this a MUST??*
>
> Also the more important question is that: what if the discontiguous area
> is
> area 0. i.e I have
> area 0 ---->area 5 ----> area 0
> should I connect area0s together using a virtual link?
> I also LABed that and it worked ok without virtual links. *is this a
> MUST
> too?*
>
>
>
> 2008/5/15 Ina&Laurean <ina.laurean@gmail.com>:
>
>> Depending on IP address space on Area 5 you may experince problems if
> you
>> want to summarize.
>> A single summary route for Area 5 won't work if you don't link the two
>> sections together.
>>
>> Laurean
>>
>>
>>
>> On Wed, May 14, 2008 at 9:31 PM, Miguel Trejo <mike.trejo@gmail.com>
>> wrote:
>>
>>> The main reason behind this working with no problem is that LSA
> doesn't
>>> carry out area information once they are injected as inter-area
> routes. So
>>> when they reach the "other"area 5 that ABR doesn't care about the
> original
>>> area this belonged to, only about the ABR that originated the route
> as the
>>> routers inside the "other" area 5 only care that this ABR knows how
> to
>>> reach
>>> the remote networks. OSPF is alink state protocol only at area level,
> when
>>> talking about routes to networks in other areas we rely on what the
> ABR
>>> says, pretty much like distance vector.
>>>
>>>
>>> On Sat, May 10, 2008 at 3:51 PM, ahmed badr <eng.ahmedbadr@gmail.com>
>>> wrote:
>>>
>>>> ok I got it. thnx
>>>>
>>>>
>>>>
>>>> 2008/5/10 Ibrahim kabir <kebramccie@live.com>:
>>>>
>>>>> yeah jason i meant OI i.e inter-area routes. The terms keep
> confusing
>>> me
>>>>> inter(btween areas) and intra(within same) area.
>>>>>
>>>>> Kabir K Ibrahim
>>>>> B.sc CCNA CCNP CCDP CCNP MCP
>>>>> +2348036477283
>>>>>
>>>>>
>>>>>> Date: Sat, 10 May 2008 08:23:43 -0600
>>>>>> From: madsen.jason@gmail.com
>>>>>> To: kebramccie@live.com
>>>>>> Subject: Re: OSPF areas
>>>>>> CC: eng.ahmedbadr@gmail.com; ccielab@groupstudy.com
>>>>>> correct, a virtual link doesn't seem appropriate here. Kebram,
> I
>>> think
>>>>> you
>>>>>> meant that the discontiguous area 5s are seeing each other as
>>>> inter-area
>>>>>> routes and not intra-area routes, right? each area 5 should end
> up
>>>> being
>>>>>> it's own unique area unless a tunnel is used.
>>>>>>
>>>>>> Jason
>>>>>>
>>>>>> On Sat, May 10, 2008 at 6:15 AM, Ibrahim kabir
> <kebramccie@live.com
>>>>> wrote:
>>>>>>> Ahmed,
>>>>>>>
>>>>>>> neva thought about this type of design at all. bt i labbed it
> and
>>> it
>>>>> worked
>>>>>>> without gre or virtual-links. The main thing to look out for
> is
>>> that
>>>>> each
>>>>>>> area
>>>>>>> should be connected to the backbone area (Area 0). and
> looking at
>>>> your
>>>>>>> diagram the condition is true. only that the discontigious
> area
>>> 5's
>>>> see
>>>>>>> each
>>>>>>> others routes as intra-area routes.
>>>>>>> Lab it and see for urself.
>>>>>>> cheers,
>>>>>>> kebram
>>>>>>>
>>>>>>>
>>>>>>>> Date: Sat, 10 May 2008 13:30:06 +0300> From:
>>>> eng.ahmedbadr@gmail.com
>>>>>>> To:
>>>>>>> ccielab@groupstudy.com> Subject: OSPF areas> > Dears,> >
>>> according
>>>> to
>>>>> the
>>>>>>> diagram below, should I link area 5 in the two sides by any>
> mean
>>> (eg
>>>>>>> tunnel)
>>>>>>> or it should work fine.> > > > Area5 Area0 Area5>
>>> -------------------
>>>>>>> ----------- ------------------> - - - - - -> -
> --R1-----------
>>>>>>> R2------------R3------------R4-- -> - - - - - ->
>>> -------------------
>>>>>>> ----------- -------------------> > >
>>>>>>>
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Next message: Salau, Yemi: "RE: OSPF areas"
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