Re: OSPF Path Selection

Hi Joe

yeah that's the way i'm interpreting it, unless advised to do so ;)

but agree really confusing as you'd think it's actually not an area 0 abr
it's crossing - technically it is

--
BR
Tony
On 27 September 2013 14:58, Joe Astorino <joeastorino1982_at_gmail.com> wrote:
> Hey Tony,
>
> Are you saying the example in Petr's article is demonstrating "rule #3"
> from the CCIE book?  That is my best guess at this point.  The thing is,
> when I read the RFC it the rules make sense.  When I read Petr's article it
> makes perfect sense.  When I read rule #3 a few nights ago I guess the way
> it was worded just did not resonate with me.
>
>
> On Fri, Sep 27, 2013 at 4:48 AM, Tony Singh <mothafungla_at_gmail.com> wrote:
>
>>
>> Edit the intra area comment clearly a poor guess.
>>
>> Ok just read Petr's PDF too he's basically referring to what we'd not
>> consider #3 to actually be #3 i.e non-zero area to non-zero area by means
>> of a VL, essentially the requirement of a type 3 LSA is still valid to
>> cross areas.
>>
>> --
>> BR
>>
>> Tony
>>
>> Sent from my iPhone on 3
>>
>> On 27 Sep 2013, at 08:40, Tony Singh <mothafungla_at_gmail.com> wrote:
>>
>> >
>> > 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]
>> >>>>>>>
>> >>>>>>> "He not busy being born is
>> >>>>>> busy dying" - Dylan
>> >>>>>>>
>> >>>>>>> Blogs and organic groups at http://www.ccie.net
>> >>>>>> [2]
>> >>>>>>
>> _______________________________________________________________________
>> >>>>>> Subscription information may be found at:
>> >>>>>> http://www.groupstudy.com/list/CCIELab.html [3]
>> >>>>>>
>> >>>>>>
>> >>>>>>
>> >>>>>> Links:
>> >>>>>> ------
>> >>>>>> [1]
>> >>>>>> http://astorinonetworks.com
>> >>>>>> [2] http://www.ccie.net
>> >>>>>> [3]
>> >>>>>> http://www.groupstudy.com/list/CCIELab.html
>> >>>>>> [4]
>> >>>>>>
>> http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/
>> >>>>>>
>> >>>>>>
>> >>>>>> Blogs and organic groups at http://www.ccie.net
>> >>>>>>
>> >>>>>>
>> _______________________________________________________________________
>> >>>>>> Subscription information may be found at:
>> >>>>>> http://www.groupstudy.com/list/CCIELab.html
>> >>>>>
>> >>>>>
>> >>>>> Blogs and organic groups at http://www.ccie.net
>> >>>>>
>> >>>>>
>> _______________________________________________________________________
>> >>>>> Subscription information may be found at:
>> >>>>> http://www.groupstudy.com/list/CCIELab.html
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>>
>
>
>
> --
> Regards,
>
> Joe Astorino
> CCIE #24347
> http://astorinonetworks.com
>
> "He not busy being born is busy dying" - Dylan
Blogs and organic groups at http://www.ccie.net