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Exam 400-007 topic 1 question 39 discussion

Actual exam question from Cisco's 400-007
Question #: 39
Topic #: 1
[All 400-007 Questions]

How many fully established neighbour relationships exist on an Ethernet with five routers running OSPF as network type broadcast?

  • A. 5
  • B. 6
  • C. 7
  • D. 10
  • E. 20
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Suggested Answer: C 🗳️

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sat_be
Highly Voted 1 year, 7 months ago
7 is correct. Assuming routers R1-R5, R1 is elected DR, R2 BDR. Fully established neighborships only occur to DR and BDR, all other neighbors are in 2Way DR other state. DR|BDRFull relationships: R2-R1 R3-R1 R3-R2 R4-R1 R4-R2 R5-R1 R5-R2
upvoted 15 times
XalaGyan
6 months, 1 week ago
very nice explanation, many thanks
upvoted 1 times
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rockin
Most Recent 3 weeks, 5 days ago
Selected Answer: B
The Scenario: 5 OSPF routers on a broadcast network. DR and BDR are elected. DROthers form adjacencies only with the DR and BDR. Full Neighbor Relationships: 1) DR forms adjacencies with: BDR (1 neighbor). 3 DROthers (3 neighbors). Total: 4 adjacencies. 2) BDR forms adjacencies with: DR (1 neighbor, counted already above). 3 DROthers (3 neighbors). Total: 4 adjacencies. 3) DROthers form adjacencies with: DR (1 adjacency each). BDR (1 adjacency each). No adjacencies with other DROthers. **Total Neighbor Relationships: Counting only unique, fully established adjacencies: DR ↔ 4 (1 BDR + 3 DROthers). BDR ↔ 4 (1 DR + 3 DROthers). Total = 4 (DR) + 4 (BDR) − 1 (DR ↔ BDR counted twice) = 6.
upvoted 1 times
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Rim007
9 months, 1 week ago
Selected Answer: C
MY choice is C, because that the most probable.
upvoted 1 times
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arumu_123
1 year, 7 months ago
n x n-1 /2 = 10
upvoted 4 times
ying162
1 year, 5 months ago
wrong formula for OSPF in broadcast domain. Valid for IBGP full mesh though
upvoted 4 times
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