Re: OSPF Path Selection

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]
>
>
Received on Fri Sep 27 2013 - 07:03:06 ART