Re: OSPF Path Selection

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
>> [2]
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>> 
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