Re: Default LSA creation

[Acee Lindem <[e-mail]>]

Mani Devarajan wrote:

>      ASBR
>       -----     ------     -----    -------
>      |     |   |      |   |     |  |       |
>      | R1  |   | R2   |   |  R3 |  |  R5   |
>      |     |   |      |   |     |  |       |
>       -----     ------     -----    -------
>        |  |I1 I2|  | |I4  I5|  |I6 I9| |
>  RIP---    -----   |  ------    -----  |
>            AREA1   |  AREA 0    AREA2  |
>                    |I3                 |I10
>                    |      -----        |
>                    |   I7|     |I8     |
>                     -----|  R4 |-------
>                    AREA0 |     | AREA2
>                           -----
>         * AREA2 is NSSA.
>
> Accee, this is question is similar to what Takenaga
> asked. But still my doubt is not cleared.
>
> So if R5 chosses the default route through R4, and
> If i bring down the I3. R4 is still a NSSA ABR so
> it will not flush the default-type7 LSA it generated
> in NSSA-2.
>
> Do you mean to say that it is the job of R5 to
> recalculate it routes through R3 or interface I3
> going down should be avoided.


Nope. Assuming I7 is still up (even though I3 is down),
R4 still is an ABR and should be originating an NSSA
type 7 default. In this situation you may want to inhibit
automatic origination of the NSSA type 7 default or take
steps to reduce the possibility that your backbone will
become partitioned.


>
>
> -Thanks in advance,
> Mani
>
>
>
>
> Acee Lindem wrote:
>
>>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
>>
>


--
Acee