Re: OSPF areas

From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Wed May 21 2008 - 09:39:43 ART

Hi Ahmed,

The RFC does not specify that you cannot have disconnected area 0s.
However, in most designs this is a requirement due to the inter-area
route selection algorithm. If an ABR has an active neighbor in area 0,
will not use inter area routes which it has learned from a non backbone
area .

Regards,

Paul.

ahmed badr wrote:
> I googled for the below and found that in a blog....Is that true?
>
> "Please note that multiple areas in ospf domain can share same id and there
> is no requirement to link all areas with same name together with virtual
> link (except area 0). Virtual-links are used to connect dis-contiguous area
> 0 into a single area and also to link all areas to backbone area. Another
> alternative for linking other areas to backbone/making backbone area
> contiguous is to use GRE tunnels. .....SESANO"
>
>
>
> 2008/5/21 ahmed badr <eng.ahmedbadr@gmail.com>:
>
>> 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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