Re: Default LSA creation

[Acee Lindem <[e-mail]>]

TAKENAGA Yoshinobu wrote:

> Hi,
>
> I have a question about creation of default lsa into stub areas.
> (This question may be a FAQ, please suggest the point)
>
> Consider this network.
>
>    area 1   :  area 0
>    (stub)   :     +--->>(AS extenal path)
>             :     |
>  |--R1--|   :  |--R4--|
>  |      |--R2--|      |
>  |          :         |
>  |      |--R3--|      |
>  |--R6--|   :  |--R5--|
>             :
>
> Here, R2 and R3 are area border router.
> R4 is AS boundary router.
> All link have same cost.
>
> In such situation, R2 and R3 will create default Type-3 LSA into area 1.
> R1 will select R2 as nexthop to the AS extenal path through R4.
>
> Now, the link state changes to this (R4 lose the link to N1).
>
>    area 1   :  area 0
>    (stub)   :     +--->>(AS Extenal path)
>             : N1  |
>  |--R1--|   :  |XXR4--|
>  |      |--R2--|      |
>  |          :         |
>  |      |--R3--|      |
>  |--R6--|   :  |--R5--|
>             :
>
> In such situation, R2 becomes a blackhole.
>
> What is the minimum requirement for R2 to create default Type-3 LSA
> into stub areas?
>   - having configuration of Area0
>   - having active interface to Area0
>   - having "Full" status neighbor in Area0
>   - other??


Hi Takenaga,

The answer is other. The minimum requirement is that the
router be an Area Border Router (ABR). Section 3.3 of RFC 2328 defines
an ABR as a router that attaches to multiple areas. In the context of
RFC 2328, area attachment requires at least one active interface in that
area. Some implementations use other definitions of what constitutes
an ABR (e.g., refer to draft-ietf-ospf-abr-alt-04.txt). However, using
a different ABR definition doesn't handle all situations. Consider R6 and
R3 in the figure below:


     area 1   :  area 0
     (stub)   :     +--->>(AS Extenal path)
              : N1  |
   |--R1--|   :  |--R4--|
   |      |--R2--|      |
   |          :         |
   |      |--R3--|      |
   |--R6--|   :  |--R5XX|


In general, a good OSPF network design will minimize the possibility
that the backbone will become partitioned.


>
> # In NSSA with Type7 LSA, same problem will take place.
>
> Regards,
>
> --
> TAKENAGA Yoshinobu, Fujitsu.
>
>


--
Acee