Re: OSPF Path Selection

After looking at Petr's fantastic article again, I actually think I get
what the rule is saying now. I just needed a different way to look at it :
) So take the topology in that article and pretend there is a loopback on
R4 of 4.4.4.4 in say area 4. If a packet wanted to get from R1 in area 2
to that loopback in area 4 I believe the following set of events would take
place. Let's assume we have the VL in place from R3 to R4 and the default
capability transit enabled

- Since R3 is now an ABR due to the VL, then it advertises a type 3 summary
LSA for 4.4.4.4 into area 2. This LSA is not ignored by R1 anymore because
it came from a router attached to area 0

- R1 would route the packet to R3
- If the path through the transit area 1 was better than the path through
the VL, the packet could get over to R4 without ever using the VL
- The packet could arrive at R4 on the area 1 interface and be routed right
to the loopback interface on R4

So through that span of events, the packet started in a nonzero area, area
2 and made it to another non-zero area (area 4) without technically ever
transiting a backbone link.

Anybody care to agree or disagree : )

On Fri, Sep 27, 2013 at 2:43 AM, 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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> >>>>
> >>>>
> >>>>
> >>>> Links:
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> >>>> [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/
> >>>>
> >>>>
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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