Re: OSPF Path Selection

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]
> >>>>>>
> _______________________________________________________________________
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> >>>>>> [4]
> >>>>>>
> http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/
> >>>>>>
> >>>>>>
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-- 
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