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Zorluk: ZorOSPFv2 Network Types and DR/BDR Selection

An administrator observes the following command output on Router R2 regarding a broadcast multiaccess Ethernet segment:

text
R2# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:35 10.1.1.1 GigabitEthernet0/0
3.3.3.3 1 FULL/BDR 00:00:32 10.1.1.3 GigabitEthernet0/0
10.10.10.10 0 FULL/DROTHER 00:00:38 10.1.1.10 GigabitEthernet0/0

The administrator executes `clear ip ospf process` simultaneously across all routers on this segment to trigger a new election. Assuming all routers re-establish OSPF neighbor relationships at the same time, which router will be elected as the new Designated Router (DR) and why?

  1. Router 3.3.3.3 will be elected as DR because Router 10.10.10.10 is ineligible due to a priority of 0, and Router 3.3.3.3 has the highest Router ID among the eligible routers.Cevap
  2. B
    Router 10.10.10.10 will be elected as DR because its higher Router ID overrides an interface priority setting of 0 during a fresh election.
  3. C
    Router 1.1.1.1 will be elected as DR because OSPF automatically preserves previous DR role assignments across simultaneous process resets.
  4. D
    Router 2.2.2.2 will be elected as DR because the router issuing the troubleshooting command takes precedence during neighbor re-establishment.

Cevap

Router 3.3.3.3 will be elected as DR because Router 10.10.10.10 is ineligible due to a priority of 0, and Router 3.3.3.3 has the highest Router ID among the eligible routers.
On multiaccess broadcast networks, OSPF DR/BDR elections evaluate interface priority first. A router configured with priority 0 is completely ineligible to become a DR or BDR. Among the remaining eligible routers (1.1.1.1, 2.2.2.2, and 3.3.3.3), all have equal priority (1), so the highest Router ID breaks the tie. Router 3.3.3.3 has the highest Router ID among eligible routers and is elected DR.

Adım Adım Çözüm

1
Evaluate OSPF interface priority values for election eligibility.
Router 10.10.10.10 has priority 0 and is disqualified from DR/BDR candidacy.
Setting the OSPF interface priority to 0 (`ip ospf priority 0`) makes a router completely ineligible to become DR or BDR.
2
Compare priority values of remaining candidate routers.
Router 1.1.1.1 (Pri 1), Router 2.2.2.2 (Pri 1), and Router 3.3.3.3 (Pri 1) are tied.
All remaining eligible routers share the default interface priority of 1.
3
Apply the tie-breaker criterion (highest OSPF Router ID).
Router 3.3.3.3 wins the DR role.
When eligible candidates have equal non-zero priority, the router with the numerically highest Router ID (3.3.3.3 > 2.2.2.2 > 1.1.1.1) is selected as DR.

Anahtar Kavram

OSPFv2 DR/BDR Election Criteria and Priority 0 Ineligibility
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OSPFv2 Network Types and DR/BDR Selection

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