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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network engineer is configuring OSPFv2 across a broadcast Ethernet network segment and verifying router behaviors. Which two statements regarding OSPFv2 Router ID determination and neighbor adjacency formation are correct?

  1. A router ID configured explicitly with the router-id command takes precedence over IP addresses assigned to loopback or physical interfaces.Answer
  2. On a broadcast network segment, two neighboring routers with an OSPF interface priority of 0 will remain in the 2-WAY neighbor state with each other.Answer
  3. C
    Configuring a new loopback interface with a higher IP address immediately preempts and updates the active OSPF router ID without restarting the process.
  4. D
    Two neighboring routers configured with different OSPF Area IDs on connecting interfaces will establish an adjacency if both interfaces use default Hello timers.

Answer

The two correct statements are that an explicitly configured router ID takes precedence over interface IP addresses, and that two routers with an OSPF priority of 0 on a broadcast network will remain in the 2-WAY state with each other.
An explicitly configured router ID using the 'router-id' command always takes precedence over loopback and physical interface IP addresses. Furthermore, on broadcast multiaccess networks, routers configured with a priority of 0 cannot become DR or BDR; two such DROTHER routers establish bi-directional communication but remain in the 2-WAY state with each other, forming FULL adjacencies only with the elected DR and BDR.

Step-by-Step Solution

1
Evaluate OSPF Router ID election hierarchy
The election order is: 1) explicit 'router-id' CLI command, 2) highest IP address on an active loopback interface, 3) highest IP address on an active non-loopback/physical interface.
Explicit configuration always overrides automatic interface IP selection.
2
Evaluate Router ID preemption rules
Router ID selection is non-preemptive once the OSPF process initializes.
Changing interface IPs or adding loopbacks will not alter the active Router ID until 'clear ip ospf process' is executed or the router reboots.
3
Evaluate OSPF neighbor states on multiaccess broadcast networks
Setting 'ip ospf priority 0' disables DR/BDR eligibility for a router, making it a DROTHER.
DROTHER routers form FULL adjacencies only with the DR and BDR. Communication between two DROTHERs remains bidirectional in the 2-WAY state.
4
Verify Hello packet parameter matching requirements
Area ID, Subnet Mask, Hello/Dead timers, and Authentication must match between neighbors.
An Area ID mismatch causes Hello packet rejection, preventing adjacency formation entirely.

Key Concept

OSPFv2 Router ID election precedence and DROTHER 2-WAY neighbor relationship dynamics
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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