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Where to configure OSPF summary?
In R1's route table, there are 4 26-bit OSPF routes -172.16.1.0/26, 172.16.1.64/26, 172.16.1.128/26 and 172.16.1.192/26. Which configuration is right to ensure the 4 routes appear as 1 summary route of 172.16.1.0/24 in R4?
R1's Route Table
A. Configure "area 1 range 172.16.1.0 255.255.255.0" on R1
B. Configure "area 1 range 172.16.1.0 255.255.255.0" on ABR-1
C. Configure "area 0 range 172.16.1.0 255.255.255.0" on ABR-1
D. Configure "area 0 range 172.16.1.0 255.255.255.0" on ABR-7
E. Configure "area 7 range 172.16.1.0 255.255.255.0" on ABR-7
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Answer: B
Explanation: You can summarize OSPF route via "Area range" or "Summary-address" command. Because the 4 routes to be summarized are OSPF internal routes, the "area-range" command should be used.
The area range command needs to be configured at Area Border Routers (ABRs). The result is that a single summary route is advertised to other areas by the ABR.
In addition the ABR can only summarize routes that originated from the area it is in.
Answer B correctly summarizes 4 routes which originated in Area 1 at Area 1's border router ABR-1.
Why is Answer D wrong? Answer D tries to summarize at the ABR-7 which borders Area 0 and Area 7. Because these 4 routes are originated from Area 1, and seen by this border-router as Inter-area route (type 3 LSA), ABR-7 can not summarize the routes originated from other areas. (In other words, ABR-7 could not remove type 3 LSA during summarization, only Type 1 LSA)