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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Two Cisco routers, R1 and R2, are connected to a shared Ethernet switch on the 10.1.1.0/24 broadcast multiaccess network running OSPFv2 in Area 0. Router R1 initializes first with an OSPF interface priority of 1 and a Router ID of 1.1.1.1. Seeing no other routers during its Wait timer, R1 elects itself as the Designated Router (DR). Ten minutes later, Router R2 is booted on the segment with its connected interface configured with an OSPF priority of 100 and a Router ID of 2.2.2.2. Which OSPF role will R2 assume on this network segment once neighbor adjacency is fully established?

  1. Backup Designated Router (BDR), while R1 remains the Designated Router (DR)Answer
  2. B
    Designated Router (DR), preempting R1 due to R2's higher OSPF priority
  3. C
    DROTHER, because R2 cannot claim a designated role after the initial election completes
  4. D
    Designated Router (DR), while R1 is demoted to BDR following preemption recalculation

Answer

Router R2 will assume the Backup Designated Router (BDR) role, while Router R1 remains the Designated Router (DR).
In OSPFv2 broadcast multiaccess networks, DR/BDR elections are non-preemptive. Once a router establishes itself as the active Designated Router (DR), it retains that role even if another router with a higher interface priority or higher Router ID subsequently joins the segment. Because R1 was already serving as DR and no BDR was present, R2 is elected as the Backup Designated Router (BDR).

Step-by-Step Solution

1
Analyze the state of the OSPF segment when R1 boots alone.
R1 boots first, waits for neighbor Hellos during the Wait timer, finds no other router, and elects itself as the active DR on the multiaccess Ethernet segment.
On a broadcast multiaccess network, if no DR is present and no other candidate responds, the first active router assumes the DR role.
2
Evaluate OSPF DR/BDR election behavior when R2 joins the network.
OSPF DR and BDR elections are non-preemptive. An active DR will not be unseated by a newly connected router, regardless of the new router's higher OSPF interface priority or Router ID.
Non-preemption prevents network instability and frequent SPF recalculations whenever routers reboot or join the segment.
3
Determine the specific role assigned to R2.
Because a DR already exists (R1) and no BDR is currently active, R2 fills the vacant BDR position.
R2 has an eligible priority (100>0100 > 0), making it eligible for the BDR role.

Key Concept

Non-preemptive nature of OSPFv2 DR/BDR elections on broadcast multiaccess networks
Estimated Time:1m 15s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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