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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Two Cisco routers are directly connected through an Ethernet switch on the 192.168.10.0/24 subnet and are currently operating in OSPFv2 Area 0 using default Ethernet interface settings. A network engineer enters the interface configuration mode on both routers and executes the command `ip ospf network point-to-point`. Which statement correctly describes the operational impact of this configuration change on the link?

  1. OSPFv2 will suppress Designated Router (DR) and Backup Designated Router (BDR) elections while preserving neighbor adjacency.Answer
  2. B
    OSPFv2 will automatically reset the interface priority to 0, causing both routers to enter the DROTHER operational state.
  3. C
    OSPFv2 neighbor adjacency will immediately drop due to a mismatch between default broadcast and point-to-point Hello and Dead timers.
  4. D
    OSPFv2 will continue electing a DR and BDR based on highest Router ID, but multicast traffic will be converted to unicast.

Answer

OSPFv2 will suppress Designated Router (DR) and Backup Designated Router (BDR) elections while preserving neighbor adjacency.
When an OSPF interface is explicitly configured as a point-to-point network type (`ip ospf network point-to-point`), OSPF recognizes that only two devices share the link. Consequently, Designated Router (DR) and Backup Designated Router (BDR) elections are bypassed, streamlining neighbor relationships while maintaining normal multicast OSPF communications and full adjacency.

Step-by-Step Solution

1
Identify default OSPF network type behavior on Ethernet interfaces
By default, Ethernet interfaces operate as Broadcast multiaccess networks, requiring DR and BDR elections.
Broadcast networks use DR/BDR roles to minimize adjacency count and LSA flooding over multiaccess media.
2
Analyze the effect of configuring `ip ospf network point-to-point`
Changing the network type to point-to-point informs OSPF that exactly two routers are connected on the segment.
Since only two routers share the link, DR and BDR elections are unnecessary and are bypassed.
3
Verify timer compatibility between default broadcast and point-to-point network types
Both broadcast and point-to-point types use a 10-second Hello timer and a 40-second Dead timer.
Because timers match on both ends, neighbor adjacencies form and remain stable in the FULL state.

Key Concept

OSPFv2 Point-to-Point Network Type Characteristics and DR/BDR Elimination
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Topics

OSPFv2 Network Types and DR/BDR Selection

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