Hello,
an ABR will by default insert every network with the exact mask from area 2 into the backbone area 0 and vice versa. The important thing is that the information about other area´s networks is inserted as OSPF LSA type 3. By using a different type than LSA 1 (router LSA) all routers within the area will know that those networks are found somewhere behind the ABR but are not directly connected to the ABR.
This is the default behaviour and nothing to worry about.
The area range command allows you now to suppress the specific networks and summarize them.
Now in case your T1 goes down and this splits your area 2 in two parts, then let each ABR attached to "his half" announce the networks for "his half" only.
The problem arises if a router claims to have networks reachable it doesn´t really have.
Lets assume the following scenario: N1-R1 - R2 - R3-N3
And lets assume R1 has network N1 = 10.1.0.0/16 attached. R3 should have network N3 10.3.0.0/16 attached. Everything will be reachable in case the exact routes will be announced.
Once I write a summary route on R1 and R3 which sy 10.0.0.0/8 is reachable through me things go wrong. Whom should router R2 believe? Loadsharing (f.e. every second packet) will not help either. The underlying reason is that because of the "area range" both routers R1 and R3 claim to have the address range of the other one as well.
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Martin
