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List:       ms-ospf
Subject:    Re: multiple areas
From:       OSPF Project Account <teospf () TEIL ! SOFT ! NET>
Date:       1998-11-05 15:14:40
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hi alex,
thank you for your reply.
        As per your statement in router running more than one OSPF, if
one Dijkstra's yield route X->A (intra-area route)
and another yield route X->B (inter-area route)
with cost (X->B) < (X->A)
Still install one route X->A right ... (the shortest path is not
installed in the kernel routing table here !., you meant just use the
preference., right ? )

Did i get you ?

thanx in advance vyas
svr

ps: X->B means route to X with next hop B


On Wed, 4 Nov 1998, Alex Zinin wrote:

> Hi,
>
> >The RFC tells that separate algorithm is run for different areas in an
> >Area border Router (case when ABR belongs to more than one Areas.)
> >Hence , there will be a separate database stored with respect to each area.
> >Link state database specific to a area (which the algorithm is
> >running) will be used to calculate  routes (whether it is inter- area, or
> >intra-area or ASext routes etc.,)
>
> correct.
>
> >
> >My question is:
> >        when Dijkstra's algorithm is run with respect to one of the
> >Area's using its LS database, it might result in a route to say some X
> >with next hop A.
> >        The same algo when run with respect to ** another ** area (which the
> >router belongs ) using its LS database, it might yield a route to X with
> >next hop B.
>
> Yes this can happen even with internal OSPF routes, but normally
> one prefix will be represented by a type1/2 LSA only in one area, as
> in OSPF one link can only belong to one area.
> Consider an ABR, having one type-2 LSA and one type-3 one, describing
> the same prefix. The first one was received from a non-backbone
> area and was originated in it. The second one was received from the
> backbone and was originated by another ABR in the area.
> As far as type-2 is always prefered over type-3, the router will choose
> the intra-area route for installation.
>
> >
> >I see this case happening with AS external LSA's learnt. Also this case
> >can happen when our ABR is an AS external router also.
>
> Case 1. -- several type-5 LSAs coming from different routers.
> In this case the preference rule is given in the RFC's 16.4 section,
> i.e. intra-area < interarea < E1 < E2   plus some details.
>
> Case 2. -- the router originates a type-5 LSA and receives another one
> from its neighbor.
> First of all we should skip self-originated LSAs from consideration, so
> we only should count other ones. If so, the choice is not between
> two OSPF routes, but between an OSPF-route and a route learned
> from another route source. This problem is usually solved by deciding
> which route source is more preferable. If you trust the source of the route
> which you used to originate your own LSA, then you will not install
> the routes derived from other LSAs. If you trust OSPF more, than you
> will install the route from another OSPF-router and most likely delete
> the one you used for LSA-originating, thus leading to flashing your LSA
> from the OSPF-domain using the MaxAge LSA flooding.
>
> HIH,
> ------------------------------------------------------------------
> Alex D. Zinin, Consultant
> CCSI #98966
> CCIE #4015
> AMT Group / ISL
> Cisco Systems Golden Certified Partner
> http://www.amt.ru
> irc: //EFNET/#cisco, //irc.msn.com/#NetCisco [Ustas]
>

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