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352-001 · Question #173

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…

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…

Layer 3 Control Plane

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

How the community answered

(33 responses)
  • A
    12% (4)
  • B
    9% (3)
  • C
    52% (17)
  • D
    27% (9)

Why each option

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.

AAdd a layer of regional Layer 3 aggregation devices, but leave the ABR function on the data center

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.

BAdd a layer of regional Layer 2 aggregation devices, but leave the ABR function on the data center

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.

CAdd a layer of regional Layer 3 aggregation devices and move the ABR function to the regionalCorrect

Regional Layer 3 devices establish new OSPF area boundaries so that Type-1 and Type-2 LSAs generated by access-layer link flaps are confined to the regional area and replaced with Type-3 summary LSAs toward the data center. Moving the ABR role to the regional tier ensures the data center backbone area receives only stable summary information, eliminating the instability caused by thousands of access devices flooding LSAs directly to data center routers.

DAdd a layer of regional Layer 2 aggregation devices and move the ABR function to the regional

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

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

Topics

#OSPF stability#ABR placement#Layer 3 aggregation#link flapping

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