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Zorluk: ZorOSPFv2 Network Types and DR/BDR Selection

A network administrator is evaluating an OSPFv2 implementation across a shared Ethernet segment (172.16.1.0/24). Router R1 initialized first on the segment and assumed the Designated Router (DR) role. Routers R2 and R3 joined subsequently. Router R4 was added last, configured with an OSPF interface priority of 200, whereas R1, R2, and R3 retain the default interface priority of 1.

Executing `show ip ospf neighbor` on Router R4 produces the following output:

text
R4# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:36 172.16.1.1 GigabitEthernet0/1
2.2.2.2 1 FULL/BDR 00:00:34 172.16.1.2 GigabitEthernet0/1
3.3.3.3 1 FULL/DROTHER 00:00:38 172.16.1.3 GigabitEthernet0/1

Based on this output and OSPF operational behavior, why has Router R4 not assumed the Designated Router role despite having a higher interface priority?

  1. OSPF DR and BDR elections are non-preemptive; once an active DR is elected, a newly connected router will not trigger a election takeover even if configured with a higher priority.Cevap
  2. B
    Router R4 is disqualified from DR election because any interface priority value greater than 100 is automatically treated as priority 0 by OSPFv2.
  3. C
    Router R1 maintains the DR position because its lower numerical Router ID (1.1.1.1) takes precedence over interface priority values during adjacency negotiation.
  4. D
    Router R4 must be configured in point-to-point network mode before high interface priority values are recognized for DR election eligibility.

Cevap

OSPF DR and BDR elections are non-preemptive; once an active DR is elected, a newly connected router will not trigger a election takeover even if configured with a higher priority.
OSPF DR and BDR election processes are strictly non-preemptive. Once a router is elected as Designated Router, it retains that position until its OSPF process reboots, its interface goes down, or the process is manually cleared. Bringing up a new router with a higher interface priority (such as 200) does not preempt the active DR.

Adım Adım Çözüm

1
Analyze the OSPF interface state and neighbor table output from Router R4.
Router R1 (Router ID 1.1.1.1) is serving as the active DR, and Router R2 (Router ID 2.2.2.2) is serving as the active BDR.
Both roles were established when R1 and R2 initially formed adjacencies on the 172.16.1.0/24 broadcast segment.
2
Evaluate the non-preemptive election mechanics of OSPFv2.
Even though Router R4 has an interface priority of 200 (which exceeds R1's priority of 1), OSPF does not preempt an existing DR or BDR.
Preemption would cause unnecessary LSA re-flooding and adjacency resets on multiaccess segments.
3
Determine how R4 could become the DR.
R4 will only become DR if the active DR and BDR fail, or if the OSPF routing processes on R1 and R2 are cleared/restarted.
Election procedures run only when the DR or BDR position becomes vacant.

Anahtar Kavram

Non-preemptive nature of OSPF DR/BDR elections on broadcast multiaccess networks
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OSPFv2 Network Types and DR/BDR Selection

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