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Difficulty: HardOSPFv2 Network Types and DR/BDR Selection

An active OSPFv2 Designated Router (DR) on a multiaccess Ethernet segment suddenly fails. Place the operational recovery events and DR/BDR re-election steps in the correct chronological order from first to last.

  1. 1Neighboring routers detect the loss of the active DR due to the expiration of the OSPF Dead Router Timer.
  2. 2The existing Backup Designated Router (BDR) automatically promotes itself to become the new DR.
  3. 3Remaining eligible routers with an interface priority greater than 0 initiate an election process to fill the vacant BDR role.
  4. 4The eligible router with the highest interface priority (or highest Router ID as a tie-breaker) is elected as the new BDR.
  5. 5DROTHER routers build FULL neighbor adjacencies with the newly promoted DR and new BDR while remaining in 2-WAY state with each other.

Answer

The correct order of events following an OSPF DR failure is: (1) Neighbor routers detect DR loss via Dead Timer expiration, (2) The current BDR immediately promotes to DR, (3) Eligible routers initiate an election for the open BDR post, (4) The candidate with the highest interface priority or Router ID is elected new BDR, and (5) DROTHER routers complete FULL adjacency with the DR and new BDR while remaining in 2-WAY state with peer DROTHERs.
When an active DR fails, OSPF routers must first detect the loss when the Dead Router Timer expires. Upon detection, the active BDR is immediately promoted to DR without holding a new DR election. With the BDR position now vacant, remaining eligible routers (having priority > 0) participate in a BDR election, where the router with the highest interface priority (or highest Router ID if priorities are equal) wins. Finally, DROTHER routers form FULL adjacencies with both the DR and new BDR while maintaining 2-WAY state with non-DR/BDR peers.

Step-by-Step Solution

1
Detect DR failure
Neighbors observe Dead Router Timer expiration.
OSPF routers on multiaccess networks wait until the Dead Timer expires without receiving Hello packets from the DR before taking action.
2
Promote active BDR
The existing BDR assumes the DR role.
OSPF avoids holding a fresh DR election; the pre-elected BDR instantly fills the DR position to ensure fast network convergence.
3
Trigger new BDR election
Eligible routers (priority > 0) initiate an election for the vacant BDR role.
Because the previous BDR was promoted to DR, the BDR position is empty and must be filled by a new election.
4
Determine election winner
The candidate with the highest interface priority (or highest Router ID) becomes the new BDR.
OSPF tie-breaking rules evaluate highest interface priority first, followed by highest Router ID.
5
Establish neighbor adjacencies
DROTHER routers transition to FULL state with the DR and new BDR while maintaining 2-WAY state with each other.
Multiaccess network design requires routers to form full adjacencies only with the DR and BDR to restrict link-state advertisement overhead.

Key Concept

OSPFv2 DR/BDR Non-Preemption and Failure Recovery Sequence
Estimated Time:2m 0s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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