From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Wed May 21 2008 - 13:55:42 ART
Oops, should have said that summary routes from a NON-BACKBONE area are
not forwarded....
Paul Cosgrove wrote:
> Hi Yemi,
>
> I wasn't trying to make a point about intra area routes vs inter area
> routes, just about the handling of inter-area (summary) routes. In the
> example where three routers form a triangle, connected using three
> different areas, each router is receiving a summary route for the one
> link it is not directly connected to. Provided each router has another
> interface in area 0, and does not have a neighbor in that area 0, it
> will still see the summary route as valid.
>
> This rule is actually contained in RFC2328:
>
> 16.2. Calculating the inter-area routes
>
> The inter-area routes are calculated by examining summary-LSAs.
> If the router has active attachments to multiple areas, only
> backbone summary-LSAs are examined.
>
> In addition, summary routes are not forwarded between areas, they are
> originated by ABRs due to network LSAs and ASBRs within a connected
> area. This limits how far routes can propagate along the chain of areas
> you described in your last email.
>
> I see that the quote you mentioned from the RFC is correct, that it does
> indeed specify that the backbone must be contiguous. In reality though
> the routers have no way to know this; they only know that they have
> received summary LSAs on a non-backbone link, and the inter-area routing
> rule above determines whether or not they use such routes.
>
> Regards,
>
> Paul.
>
>
> Salau, Yemi wrote:
>> I agree, Intra-Area route is preferred to inter-area route, right?
>>
>> But what I'm saying is that, if you have Area0---Area2----Area0, and
>> there is say x.x.x.0/24 in one Area0, and y.y.y.0/24 in another. Will
>> the Middle Area2 router(s) still be able to ping x.x.x.0/24 &
>> y.y.y.0/24? Will the Area0 Router on the Right be able to ping
>> y.y.y.0/24 on Left Area0's Router? Will the left Area0 Router still be
>> able to ping the right Area0's Router's x.x.x.x0/24?
>>
>> Ideally I will love to use this:
>>
>> R1-----R2----R3-----R4
>>
>> Say R1 is running Area0 to R2
>> R2 is then running Area2 to R3
>> & Finally R3 is running another Area0 to R4?
>>
>> After testing/labbing this scenario, I discovered R4 can't ping R2's
>> interface in Area2, likewise R1 can't ping R3's interface in Area2
>> Similarly, R2 can't ping R4's interface in Area0, likewise, R3 can't
>> ping R1's interface in Area0.
>>
>> Looking deeper into the OSPF Database, on R1, you wouldn't find any link
>> state entry in OSPF database for the R3/R4 network lan. Similarly, you
>> wouldn't find any link state entry for R1/R2 network lan on R4. Could we
>> then say that this is down to R1 "filtering" that entry off its OSPF
>> database because of inter-area route?
>>
>> By the way, R2 has Router & Summary Network Link states for the R3/R4
>> network, can ping R3's interface on this network, but can't ping R4's
>> Similary, R3 has Router & Summary Network Link states for the R1/R2
>> network, can ping R2's interface on this network, but can't ping R1's
>>
>> Maybe I'm not using the correct English to describe this, but I'm yet to
>> find an RFC (still searching though, please forward me a link if you
>> find one) that link the problems of discontiguos area0 to inter/intra
>> area route preference. But what RFC did say is that "OSPF Area0 is
>> responsible for distributing routing information btw non-backbone areas,
>> the backbone must be contiguos. However it nee not be physically
>> contiguous"
>>
>> Many Thanks.
>>
>> Yemi Salau
>>
>> -----Original Message-----
>> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>> Paul Cosgrove
>> Sent: Wednesday, May 21, 2008 2:41 PM
>> To: Salau, Yemi
>> Cc: ahmed badr; Ina&Laurean; Miguel Trejo; Cisco certification
>> Subject: Re: OSPF areas
>>
>> 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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