Re: Inter-area routes reachable via multiple areas

[Acee Lindem <[e-mail]>]

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]
>>>>
>>>>__________________________________
>>>>Do you Yahoo!?
>>>>Yahoo! SiteBuilder - Free, easy-to-use web site design software
>>>>http://sitebuilder.yahoo.com
>>>>
>>>
>>>
>>>--
>>>Acee
>>
>
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--
Acee