From: steveaggie@gmail.com
Date: Sat Mar 15 2008 - 14:02:27 ARST
This is interesting. I would have thought the behavior described below
would be accurate too. However, I am seeing something different. I have R4
neighbored to R3 in area 0. R4 also has a route to R3 through area 38 + an
OSPF/EIGRP redistribution router (SW2). R3's loopback is advertised into
EIGRP and EIGRP is redistributed into OSPF on R3.
So R4 has an E2 route through area 0 to R3 Lo0. It also has an E2 route
through area 38 to R3 Lo0.
When I shut down R4's interface to area 38, it reaches R3 through area 0.
When I bring the interface back up it selects the route through area 38.
There are several more hops through area 38, so it is strange. Although R4
shouldn't know the difference since it's an E2 route.
I am not sure why this would be. Is the higher router ID of the area 38
neighbor breaking the tie?
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of S
Malik
Sent: Saturday, March 15, 2008 10:10 AM
To: Paul Cosgrove
Cc: Mike Haddad; ccielab@groupstudy.com
Subject: Re: Route selection for Inter Area routes
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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