Re: OSPF Path Selection

For what it's worth I totally agree as we're transiting through area 0 and the newly established ABR (after a VL has been established to a genuine area 0 ABR) to exit into an say fir example O E2 destination.....

I think by #3 they mean O intra this is my only thinking, but for OIA we'd have to traverse an area 0 ABR for a non zero area to get to another non zero area i.e it would have to receive a type 3 LSA in the first place from the ABR.

--
BR
Tony
Sent from my iPhone on 3
On 27 Sep 2013, at 07:43, Joe Astorino <joeastorino1982_at_gmail.com> wrote:
> With the default capability transit all you are doing is taking a transit area to get to area 0 instead of taking a VL through the same transit area. In both cases you still end up in area 0 then pass through area 0 to get to the other nonbackbone area.
> 
> 
> Sent from my iPhone
> 
>> On Sep 27, 2013, at 2:41 AM, Joe Astorino <joeastorino1982_at_gmail.com> wrote:
>> 
>> 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]
>>>>>> 
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>>>>> busy dying" - Dylan
>>>>>> 
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>>>>> 
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