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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

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

  • Interface configured with `ip ospf network broadcast`Performs DR/BDR elections and uses default Hello/Dead timers of 10s and 40s.
  • Interface configured with `ip ospf network point-to-point`Establishes neighbor adjacencies without electing a DR/BDR using 10s/40s timers.
  • Interface configured with `ip ospf priority 0` on a multiaccess linkPermanently remains a DROTHER and is excluded from DR/BDR election.
  • Router with priority 255 added to a link with an active, established DRFails to take over the DR role due to the non-preemptive nature of OSPF elections.

Answer

Interface configured with `ip ospf network broadcast` matches 'Performs DR/BDR elections and uses default Hello/Dead timers of 10s and 40s.'
Interface configured with `ip ospf network point-to-point` matches 'Establishes neighbor adjacencies without electing a DR/BDR using 10s/40s timers.'
Interface configured with `ip ospf priority 0` on a multiaccess link matches 'Permanently remains a DROTHER and is excluded from DR/BDR election.'
Router with priority 255 added to a link with an active, established DR matches 'Fails to take over the DR role due to the non-preemptive nature of OSPF elections.'
Each OSPF interface setting dictates specific operational properties: Broadcast links use 10s/40s timers and elect DR/BDR; Point-to-Point links use 10s/40s timers without electing DR/BDR; Priority 0 prevents any DR/BDR candidacy resulting in DROTHER status; and DR/BDR elections are non-preemptive, preventing new higher-priority routers from taking over an active DR role.

Step-by-Step Solution

1
Analyze OSPF network type behaviors regarding DR/BDR election requirement.
Broadcast networks elect a DR and BDR to reduce adjacency overhead over multiaccess media. Point-to-point networks connect exactly two routers and omit the DR/BDR election process entirely.
DR/BDR selection depends on the underlying OSPF network type setting.
2
Evaluate the effect of interface priority configuration.
Setting `ip ospf priority 0` explicitly removes a router from participating in DR or BDR election, locking its state as DROTHER.
Priority values range from 0 to 255, where 0 represents absolute exclusion from election eligibility.
3
Determine election preemption behavior when topology changes occur.
OSPF elections are non-preemptive; an existing DR retains its role even if a router with a higher priority (such as 255) joins the network after election completion.
Non-preemption prevents network instability and continuous routing recalculations upon router reboots or link flaps.

Key Concept

OSPFv2 Network Types, Interface Priority, and Election Preemption Rules
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Topics

OSPFv2 Network Types and DR/BDR Selection

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