From: S Malik (ccie.09@gmail.com)
Date: Sat Mar 15 2008 - 13:09:44 ARST
Paul/Mike,
Thanks for your assistance.
I found exactly the same behaviour. A route received by a neighbor in BB is
always selected regardless of metric. Once I create a virtual link through
area 10 and manipulate metrics then route is chosen through router R3.
I think, I was not clear that BB route is always preferred. Can any one
point me to any link explaining this behaviour please. Most of docs explain
only intra-area, inter-area, E1, E2 only.
I love this forum with great participants.
Malik
On Sat, Mar 15, 2008 at 5:50 AM, Paul Cosgrove <paul.cosgrove@heanet.ie>
wrote:
> When calculating the best inter-area route, an OSPF ABR which has a
> neighbor in the backbone will not consider any summary LSAs learned from
> a non-backbone area. If it does not have a neighbor in the backbone,
> then it will accept the summary LSAs from another area. Cost is checked
> later.
>
> Paul.
>
> Mike Haddad wrote:
> > Hello Malik,
> >
> > OSPF uses SPF algorithm to calculate the route to destinations. SPF is
> > calculated in three phases. The first is the calculation of intra-area
> routes
> > by building the shortest path tree for each attached area. The second
> phase
> > calculates the inter-area routes by examining the summary LSAs and the
> last
> > one examines the AS-External-LSAs to calculate the routes to the
> external
> > destinations.
> >
> > In your example, R2 will perform the SPF calculation on both received
> > summary routes and will choose the one that has the lowest metric =>
> shortest
> > path. You can manipulate the route selection by many ways and one is by
> > increasing the OSPF Cost on the interface going to R3 so it uses R1 for
> the
> > summary.
> >
> > Hope this helps,
> >
> > Regards,
> >
> >> Date: Sat, 15 Mar 2008 01:53:08 -0400> From: ccie.09@gmail.com> To:
> >>
> > ccielab@groupstudy.com> Subject: Route selection for Inter Area routes>
> >
> > All,> I would appreciate if some one can explain how inter area routes
> are>
> > selected in the following scenario> > R2 is receiving type-3 summary
> routes
> > from an ABR R1 in back bone area and> receiving the same route from R3
> through
> > area say 10.> > > R2 & R3 are neighbors in area 10 and they both are
> part of
> > back bone also.> > Thanks> Malik> >
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