Re: Inter-area routes reachable via multiple areas

[Acee Lindem <[e-mail]>]

Liu Yu wrote:
>     Acee :
>
> Please read the following :

       Liu  :

   Then don't implement RFC 3509 since by definition it
   deviates from RFC 2338.




>
> RFC 2328: OSPF
>
> Section 16.8 Equal-cost multipath
> "
> Each one of the multiple routes will be       of the same type
>       (intra-area, inter-area, type 1 external or type 2 external),
>       cost, and will have the same associated area.  However, each
>       route may specify a separate next hop and Advertising router."
>
> Thanks!
>
> ----- Original Message -----
> From: "Acee Lindem" <[e-mail]>
> To: <[e-mail]>
> Sent: Wednesday, September 17, 2003 3:13 AM
> Subject: Re: Inter-area routes reachable via multiple areas
>
>
>
>>Liuyu wrote:
>>
>>>I have read RFC 3509:Alternative Implementations of OSPF Area Border Routers
>>>
>>>But it still does not explicitly state how to  choose a inter-area route from multi associated areas.
>>>
>>>For example:
>>>R3 attaches to Area 2 and Area 1, but it is not a ABR acrroding to RFC 3509.
>>>When R3 caculate the route to inter-area route Net K, it will find one with next hop to R2
>>>with metric equal to 4 in Area 1 and the other one with next hop set to R4 with the same metric.
>>>
>>>How will R3 install the inter-area route Net K in its routing table?  Huawei's VRP will choose the latest
>>>caculated route.
>>>
>>>As you konw , the ECMP in OSPF must  be the same type and in the same area.
>>
>>Note that RFC 3509 is an informational RFC and not a standard. Therefore, you
>>shouldn't treat it as such. In this situation, I see no reason why you couldn't
>>install an ECMP route.
>>
>>
>>
>>>Thanks a lot!
>>>
>>>                      .        Backbone         .
>>>                     .           Net K           .
>>>                     .   ---------------------   .
>>>                      .   |1               1|   .
>>>                       ..+--+.............+--+..
>>>                       ..|R1|.....    ....|R4|..
>>>                      .  +--+     .  .    +--+  .
>>>                      .   1|      .  .     /4   .
>>>                      .    |    2 +--+ 4  /     .
>>>                      .    |    +-|R3|---+      .
>>>                      .   1|   /  +--+\4        .
>>>                      .  +--+ /   .  . \ 4 +--+ .
>>>                      .  |R2|/2   .  .  +--|R5| .
>>>                      .  +--+     .  .     +--+ .
>>>                      .   |       .  .       |  .
>>>                      . --------- .  . -------- .
>>>                      .   net N   .  .  net M   .
>>>                      .           .  .          .
>>>                      .  Area 1   .  .  Area 2  .
>>>                       ...........    ..........
>>>
>>>
>>>----- Original Message -----
>>>From: "Vivek Dubey" <[e-mail]>
>>>To: <[e-mail]>
>>>Sent: Wednesday, September 10, 2003 6:13 PM
>>>Subject: Re: Inter-area routes reachable via multiple areas
>>>
>>>
>>>The case should not occur in case of standard ABR implementations for inter-area route calculations.
>>>CISCO/IBM ABR implementations might lead to such scenario.
>>>
>>>Vivek
>>>
>>>
>>>
>>>
>>>On Wed, 10 Sep 2003 Liu Yu wrote :
>>>
>>>
>>>>Hi! all
>>>>
>>>>I notice that the clause in  RFC 2328 16.4 (3) is just for external route.
>>>>How about "inter-area routes"ï¼Y RFC 2328 does not explicitly state this
>>>>one! Just choose the "the path whose associated area has the
>>>>largest OSPF Area ID "ï¼Yï¼Y Maybe every one has his own implentation.
>>>>
>>>>
>>>>thanks!
>>>>
>>>>
>>>>----- Original Message -----
>>>>From: "Acee Lindem" <[e-mail]>
>>>>To: <[e-mail]>
>>>>Sent: Friday, August 01, 2003 5:46 AM
>>>>Subject: Re: ASBR reachable via multiple areas
>>>>
>>>>
>>>>
>>>>
>>>>>Rob,
>>>>>
>>>>>You are correct - I missed that clause in 16.4 (3) when
>>>>>re-reading. I should have checked my implementation before
>>>>>replying. I can't envision why installing an equal-costs
>>>>>routes through multiple areas using the same 16.4.1 selection
>>>>>rules could cause a routing loop or other problem.
>>>>>
>>>>>Thanks,
>>>>>Acee
>>>>>
>>>>>ROB PATH wrote:
>>>>>
>>>>>
>>>>>>--- Acee Lindem <[e-mail]> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>>Igor Miroshnik wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>All,
>>>>>>>>
>>>>>>>> RFC 1583 states: "AS boundary router's routing
>>>>>>>
>>>>>>>table entry must indicate a set of paths which
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>utilize a single area." (16.1)
>>>>>>>> Why not utilize all equal-cost paths through
>>>>>>>
>>>>>>>different areas?
>>>>>>>
>>>>>>>Hi Igor,
>>>>>>>
>>>>>>>I believe you can calculate equal cost paths through
>>>>>>>multiple
>>>>>>>areas - read section 16.4 and 16.4.1 in RFC 2328
>>>>>>>(which
>>>>>>>obsoletes RFC 2178 which obsoletes RFC 1583).
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>The section 16.4 in RFC 2328 does not explicitly state
>>>>>>that equal cost paths through multiple areas could be
>>>>>>utilized (say for load balancing purposes).
>>>>>>
>>>>>>It rather states that when there are multiple
>>>>>>least cost paths available in the routing
>>>>>>table, the path whose associated area has the
>>>>>>largest OSPF Area ID is chosen.
>>>>>>
>>>>>>Also section 16.4.1 refers to section 16.4 for
>>>>>>choosing a path based on cost, when there are
>>>>>>multiple paths of equal highest preferences.
>>>>>>
>>>>>>So it essentially means that multiple equal cost
>>>>>>inter-area paths are not utilized.
>>>>>>
>>>>>>Thanks,
>>>>>>   Rob.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Igor
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>--
>>>>>>>Acee
>>>>>>
>>>>>>
>>>>>>
>>>>>>=====
>>>>>>Rob Path
>>>>>>E-Mail : [e-mail]
>>>>>>
>>>>>>__________________________________
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>>>>>>
>>>>>
>>>>>
>>>>>--
>>>>>Acee
>>>>
>>>___________________________________________________
>>>Medicine meets Marketing; Dr. Swati Weds Jayaram.
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>>
>>--
>>Acee

--
Acee