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Exams352-001Questions#173
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352-001 · Question #173

352-001 Question #173: Real Exam Question with Answer & Explanation

The correct answer is C: Add a layer of regional Layer 3 aggregation devices and move the ABR function to the regional. Adding a regional Layer 3 aggregation tier and moving the OSPF ABR function to that tier creates area boundaries that contain access-layer link flap LSAs within regional areas, preventing them from flooding the data center backbone. This hierarchical restructuring limits SPF reca

Question

In a large enterprise network with multiple data centers and thousands of access devices, OSPF is becoming unstable due to link flapping. The current design has the access devices multihomed to large aggregation routers at each of the data centers. How would you redesign the network to improve stability?

Options

  • AAdd a layer of regional Layer 3 aggregation devices, but leave the ABR function on the data center
  • BAdd a layer of regional Layer 2 aggregation devices, but leave the ABR function on the data center
  • CAdd a layer of regional Layer 3 aggregation devices and move the ABR function to the regional
  • DAdd a layer of regional Layer 2 aggregation devices and move the ABR function to the regional

Explanation

Adding a regional Layer 3 aggregation tier and moving the OSPF ABR function to that tier creates area boundaries that contain access-layer link flap LSAs within regional areas, preventing them from flooding the data center backbone. This hierarchical restructuring limits SPF recalculations to the affected region.

Common mistakes.

  • A. Adding regional Layer 3 devices but keeping the ABR at the data center leaves data center routers still processing all access-layer Type-1 and Type-2 LSAs, so link flap instability is not contained at the regional boundary.
  • B. Regional Layer 2 devices do not participate in OSPF routing and cannot form area boundaries, meaning access-layer topology changes still propagate as Layer 3 LSAs directly to the data center aggregation routers.
  • D. Layer 2 devices cannot host the OSPF ABR function because ABR is a Layer 3 OSPF construct requiring full routing plane participation; placing the ABR role on a Layer 2 device is not technically feasible.

Concept tested. OSPF hierarchical redesign with regional ABR for stability

Reference. https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html

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