300-410 ENARSI: Implementing Cisco Enterprise Advanced Routing and ServicesDemo

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QuestionQ531

Layer 3 Technologies

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Refer to the exhibits. The SanFrancisco and Boston routers select slower links to reach one another even though the direct links are up.

Which configuration resolves the issue?

  • A All Routers router ospf 1 auto-cost reference-bandwidth 100
  • B SanFrancisco Router router ospf 1 auto-cost reference-bandwidth 1000
  • C Boston Router router ospf 1 auto-cost reference-bandwidth 1000
  • D All Routers router ospf 1 auto-cost reference-bandwidth 1000
Explanation

OSPF derives interface cost from the reference bandwidth divided by the interface bandwidth. A 1000-Mb/s reference bandwidth assigns a cost of 10 to a 100-Mb/s link and a cost of 1 to a 1-Gb/s link, making the two 1-Gb/s hops through Dallas appear cheaper than the direct 100-Mb/s link. Setting auto-cost reference-bandwidth 100 consistently on every router gives the direct link a cumulative cost of 1 and the Dallas path a cumulative cost of 2, so OSPF selects the direct path.

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