Re: Inter-area routes reachable via multiple areas

[Liuyu <[e-mail]>]

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.

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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