Are you sure that the B1 Network Summary LSA had a lower cost then the B2
Summary LSA? According to how I understood your topology, B1 should
advertise
a cost of 65 to reach network C and B2 should advertise a cost of 2. So they
should both use the T1 link to B2 to exit the area. The reason for this is
that because routers B1 and B2 both have intra-area paths to reach network C
they will always select them, even if a lower cost inter-area route through
the backbone exists. For this reason, B1 will select the higher cost Area 2
intra-area link to C1 over the T1 rather then use the lower cost inter-area
link through the Fast-Ethernet interface to B2 and then on to C2 via the T3.
"Ian Schorr" wrote:
> 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
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=22277&t=22268
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