Re: OSPF Path Selection

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]
>>>>>
>>>>> "He not busy being born is
>>>> busy dying" - Dylan
>>>>>
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Received on Fri Sep 27 2013 - 02:43:45 ART