Re: OSPF Path Selection

In my mind no because the stated rule 3 says for "a path crossing areas" "take the shortest path to the destination without crossing area 0"

With a virtual link scenario, you ride the VL which is in area 0 to an ABR. For a router in a nonzero area to reach a route in another nonzero area, even with the virtual link you still pass through area 0 at some stage.

Say you have area3---area0---area1---area2
You would build a VL from area 2 to area 0 transmitting through area 1. If a packet wants to get to area 3 from area 2 , it rides an area 0 link to the backbone (the VL) first (rule 1) Then it would take the shortest path through area 0 (rule 2)

Once I to area 0 though I don't see how it would get to area 3 "without crossing area 0"

Sent from my iPhone

> On Sep 27, 2013, at 1:59 AM, Tony Singh <mothafungla_at_gmail.com> wrote:
>
>
> The non-zero router becomes an ABR when it connects via a VL into an area 0 router.
>
> So technically is this really point 3?
>
> --
> BR
>
> Tony
>
> Sent from my iPhone on 3
>
>> On 27 Sep 2013, at 06:26, Joe Astorino <joeastorino1982_at_gmail.com> wrote:
>>
>> Yes of course, but as we know the VL is just a link in area 0 so that is not really what I'm getting at. There is also the case with the default capability transit where you can ride a transit area INTO the backbone instead of the VL but one way or another for inter area traffic you end up in the backbone
>>
>> Sent from my iPhone
>>
>>> On Sep 27, 2013, at 1:03 AM, daniel.dib_at_reaper.nu wrote:
>>>
>>> Hi Joe!
>>>
>>> This could happen if you have a virtual link between ABRs
>>> meaning that you have something Like Area 0 - Area 1 - Area 2. Check
>>> this INE blog post for the full info:
>>>
>>>
>>> http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/
>>> [4]
>>>
>>> Regards Daniel
>>>
>>> CCIE #37149
>>>
>>> 2013-09-27 06:17 skrev Joe
>>> Astorino:
>>>
>>>> So this has actually been bothering me now for YEARS. In
>>> the CCIE RS Exam
>>>> Certification Guide, there is a paragraph that goes
>>> something like this:
>>>>
>>>> *OSPF has specific rules for selecting a path
>>> that crosses areas. *
>>>>
>>>> *1) Take the shortest path to area 0.
>>>> 2)
>>> Take the shortest path across area 0 without traversing a nonzero
>>> area.
>>>> 3) Take the shortest path to the destination without traversing
>>> area 0.*
>>>>
>>>> This has always been somewhat vague and even disturbing to
>>> me. It's
>>>> seemingly vague and no other explanation is given about this
>>> process. Rule
>>>> 1, take the shortest path to area 0 makes sense. Once
>>> you get to the
>>>> backbone area, rule #2 even makes sense. But rule #3
>>> has never and does not
>>>> make sense to me
>>>>
>>>> So far as I recall, an
>>> OSPF ABR will never accept type 3 summary LSA
>>>> information from a
>>> non-backbone area. In other words, If an ABR receives
>>>> inter-area
>>> routing information for a non-backbone area from a non-backbone
>>>> area
>>> it is ignored. This makes sure that inter area routing information is
>>> only learned from the backbone area, and is also a loop prevention
>>> mechanism. Further, in my mind it guarantees that all inter-area
>>> traffic
>>>> must transit the backbone.
>>>>
>>>> With that being said, can
>>> anybody think of ANY case EVER where rule #3 is
>>>> even valid? How would
>>> it ever be possible for inter-area traffic to get to
>>>> a destination
>>> without traversing area 0?
>>>>
>>>> --
>>>> Regards,
>>>>
>>>> Joe Astorino
>>>> CCIE
>>> #24347
>>>> http://astorinonetworks.com [1]
>>>>
>>>> "He not busy being born is
>>> busy dying" - Dylan
>>>>
>>>> Blogs and organic groups at http://www.ccie.net
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>>> [4]
>>> http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/
>>>
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Received on Fri Sep 27 2013 - 02:41:12 ART