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Difficulty: HardOSPFv2 Network Types and DR/BDR Selection

Match each OSPFv2 interface scenario or configuration on the left to its corresponding election behavior or timer characteristics on the right.

  • GigabitEthernet interface manually configured with 'ip ospf network point-to-point'Uses 10-second Hello / 40-second Dead timers and dynamically forms adjacencies without electing a DR or BDR.
  • Ethernet interface configured with 'ip ospf priority 0' on a broadcast multi-access segmentRemains in the DROTHER state and is completely excluded from participating in DR/BDR elections regardless of Router ID.
  • Physical main interface configured for Frame Relay with default OSPF network type settingsDefaults to Non-Broadcast (NBMA) mode using 30-second Hello / 120-second Dead timers and requires manual unicast neighbor statements.
  • Interface with 'ip ospf priority 200' connected to an active broadcast segment where a router with priority 100 is already the operational DRDoes not preempt the operational DR upon initialization, assuming BDR or DROTHER status until an election event occurs.

Answer

The correct pairings are: 1) GigabitEthernet interface with 'ip ospf network point-to-point' matches 'Uses 10-second Hello / 40-second Dead timers and dynamically forms adjacencies without electing a DR or BDR.'; 2) Ethernet interface with 'ip ospf priority 0' matches 'Remains in the DROTHER state and is completely excluded from participating in DR/BDR elections regardless of Router ID.'; 3) Frame Relay main interface with default OSPF network type matches 'Defaults to Non-Broadcast (NBMA) mode using 30-second Hello / 120-second Dead timers and requires manual unicast neighbor statements.'; 4) Interface with priority 200 added to a segment with an active DR (priority 100) matches 'Does not preempt the operational DR upon initialization, assuming BDR or DROTHER status until an election event occurs.'
Each OSPF network type and interface parameter dictates distinct neighbor discovery methods, timer values, and election roles. Converting a broadcast interface to point-to-point maintains 10-second timers while removing DR/BDR election. Setting priority to 0 explicitly forces a DROTHER state. Default Frame Relay main interfaces employ NBMA mode with 30-second timers requiring unicast neighbor commands. Finally, introducing a high-priority router to a segment with an existing active DR honors OSPF's non-preemptive election rule.

Step-by-Step Solution

1
Analyze OSPF network type override on broadcast interfaces.
Configuring 'ip ospf network point-to-point' on a broadcast interface (such as GigabitEthernet) instructs OSPF to treat the link logically as point-to-point. This retains the 10/40 second timers but suppresses DR/BDR election.
Point-to-point interfaces only connect two routers, making DR/BDR election unnecessary.
2
Evaluate the effect of interface priority 0.
An OSPF interface priority of 0 explicitly disqualifies the router interface from becoming a DR or BDR, locking its state into DROTHER.
Priority 0 is a reserved value designed specifically to prevent specific routers from taking on DR/BDR responsibilities.
3
Determine default behavior on Non-Broadcast Multi-Access (NBMA) media.
Frame Relay main interfaces default to the Non-Broadcast (NBMA) OSPF network type, which has 30-second Hello / 120-second Dead timers and does not send multicast Hello packets.
NBMA interfaces support multiple access without native broadcast/multicast capability, requiring static unicast neighbor configuration.
4
Analyze non-preemptive DR/BDR election mechanics.
When a router boots or connects to an existing multi-access network with an established DR, it accepts the existing DR and BDR regardless of its own interface priority.
DR/BDR elections in OSPF are non-preemptive to maintain routing stability and prevent unnecessary adjacency updates.

Key Concept

OSPFv2 Network Types, Timers, Priority Exclusions, and Non-preemptive Election Behavior
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Topics

OSPFv2 Network Types and DR/BDR Selection

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