OSPF (Open Shortest Path First) calculates the shortest path to area 0
first, then the shortest path to the next area boundary via intra-area
routes. The best illustration of this was in the Networkers 2000 CCIE Power
Session presentation
http://www.ieng.com/networkers/nw00/pres/3304/3304_c1_sec2.pdf
p 16
hth...
-e-
----- Original Message -----
From: "Ian Schorr"
To:
Sent: Friday, October 05, 2001 1:49 PM
Subject: OSPF inter-area learned routes [7:22268]
> Okay, I've been beating my head over this one for a week, and finally give
> up:
>
> I'm trying to understand the logic behind OSPF's and Cisco's routing
> selection behavior in this instance:
>
> I have 6 routers, segmented into 3 areas - areas 0, 1, and 2.
>
> Routers A1 and A2, which sit in area 1, have fast ethernet interfaces on
the
> same network (we'll call this "network A"), have formed adjacencies with
> eachother across this network, and seem to be exchanging routes properly.
>
> Routers B1 and B2 are in areas 0, 1, and 2. Again, each router has a fast
> ethernet interface on the same network ("network B"), which is in area 0.
> B1 has a T3 link to A1, and this numbered link is in area 1. B2 has a T1
> link to A2, and this numbered link is in area 1.
>
> Routers C1 and C2 sit in area 2, and have fast ethernet interfaces on the
> same network ("network C"). B1 has a T1 link to C1, and B2 has a T3 link
to
> C2.
>
> Pretty much everything else is default, including link metrics, summary
> metrics, etc. No summarization is taking place.
>
> Now, I'm confused, because if I do "show ip route" on router A1 or A2 for
> network C, the selected inter-area route for network C points across the
T1
> link to B2, as opposed to the lower-cost path across the T3 to B1 (then
> across the FE link to B2, and across the T3 link to C2)! If I do a "show
ip
> ospf database summary" for network C, I do see that I've received summary
> routes for network C from both B1 and B2, and that B1 has a lower metric,
> but it seems that the A routers select the path across to B2 anyway.
>
> I assume that this has something to do with the fact that the core routers
> are both backbone routers AND ABRs for the other areas, and that LSAs for
> area 1 don't pass across the fast ethernet interface (instead, I expect to
> see summary-LSAs for the area 1 passing across area 0, then being
> re-summarized as available paths back to area 1), but I suspect that
there's
> just something fundamental about OSPF that I don't understand. Can
anybody
> explain to me why the better-metric path isn't being selected in this
case?
>
> Thanks,
> Ian Schorr
> CCNP
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