A customer requires high availability, so the network architect is planning two area border (ABRs)
for each non-backbone area in the Open Shortest Path First (OSPF) solution. What ensures a
loop-free routing environment that meets the customer needs?
A.
All ABRs have at least one interface in area 0, and that interface has an IP address that is
outside of the range of any summaries for area 0
B.
Each ABR in an area advertises the same summary routes for the area, and each ABR has a
null route that matches those summaries
C.
Only one ABR advertise a summary route for each area. For areas with multiple ABRs, each
area can include two summaries ranges, and each ABR is configured with one of those ranges
D.
All ABRs in an area apply consistent path costs for their summary routes
i think it is A… here is the reason for loop prevention:
http://networkengineering.stackexchange.com/questions/6329/why-must-all-areas-connect-to-an-area-0-backbone-in-ospf
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a
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Answer: C
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A
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A is wrong , because all ABRS neednot have one interface in area 0 when using virtual links
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Correct ans is B (Ref Pg no : 398)
The ABRs simply need null routes that match each summary route. Then, instead of accepting the other ABRs summary route, then ABR uses its null route.
HP Switches automatically create such a route when the network administrator configures an area summary, which prevents one ABR from accepting summary route from the another ABR.
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Answer:B
OSPF accept multiple ABR with route summarization of each one.
It’s interesting to look at these summary routes on the ABR, which is responsible for doing all of the summarization. The ABR also includes the summary routes, but they aren’t real routes, so it simply points them to its Null0 interface called discard routes. They help to prevent routing loops during summarization
Benefits of OSPF Summarization:
1) Reduce the number of routing table entries.
2) Reduce Type3 flooding and saves router CPU resource.
3) Summarization can also be used for route manipulation by using longest match prefix concept
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