Re: Inter-area routes reachable via multiple areas

[Liu Yu <[e-mail]>]

    Acee :

Please read the following :

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]
> >>>>
> >>>>__________________________________
> >>>>Do you Yahoo!?
> >>>>Yahoo! SiteBuilder - Free, easy-to-use web site design software
> >>>>http://sitebuilder.yahoo.com
> >>>>
> >>>
> >>>
> >>>--
> >>>Acee
> >>
> >
> > ___________________________________________________
> > Medicine meets Marketing; Dr. Swati Weds Jayaram.
> > Rediff Matchmaker strikes another interesting match !!
> > Visit http://rediff.com/matchmaker?2
> 
> --
> Acee