I sketched your network and it made me wonder if the path you think is
"shortest" really is shortest.
To get to the C routers in Area 2, the Area 1 A routers have four possible
paths:
1. A1---T3---B1---B2---T3---C2
2. A1---T3---B1---T1---C1
3. A2---T1---B2---T3---C2
4. A2---T1---B2---B1---T1---C1
Which ones looks shortest?
I think you were concerned that the A routers were using the T1 link
instead of the T3 link, but that's OK as long as they go out the T3 from
B2, which I think you said that they do. In other words, if they use route
3 instead of route 4 in my list above, there's no problem??
Priscilla
At 04:49 PM 10/5/01, 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
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=22291&t=22268
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