Re: OSPF Path Selection

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

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> 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
> [2]
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> http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/
>
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Received on Fri Sep 27 2013 - 01:26:32 ART