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:
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