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Zorluk: ZorOSPFv2 Network Types and DR/BDR Selection

Four Cisco routers (HQ-R1, HQ-R2, HQ-R3, and HQ-R4) are connected to a shared GigabitEthernet switch on the broadcast multiaccess network 10.200.1.0/2410.200.1.0/24. All routers are powered on simultaneously with default OSPFv2 settings except for interface priority values. The interfaces are configured as follows:

- HQ-R1: `ip ospf priority 0`, Loopback0: 192.168.100.1/32192.168.100.1/32, Gi0/0: 10.200.1.1/2410.200.1.1/24
- HQ-R2: `ip ospf priority 10`, Loopback0: 10.10.10.10/3210.10.10.10/32, Gi0/0: 10.200.1.2/2410.200.1.2/24
- HQ-R3: `ip ospf priority 20`, Loopback0: 1.1.1.1/321.1.1.1/32, Gi0/0: 10.200.1.3/2410.200.1.3/24
- HQ-R4: `ip ospf priority 20`, Loopback0: 2.2.2.2/322.2.2.2/32, Gi0/0: 10.200.1.4/2410.200.1.4/24

Which router role assignment correctly describes the outcome of the DR/BDR election on this Ethernet segment?

  1. HQ-R4 is elected as DR, and HQ-R3 is elected as BDR.Cevap
  2. B
    HQ-R1 is elected as DR because it possesses the highest loopback IP address (192.168.100.1).
  3. C
    HQ-R3 is elected as DR, and HQ-R4 is elected as BDR.
  4. D
    HQ-R2 is elected as DR because HQ-R3 and HQ-R4 tie on priority and forfeit the election.

Cevap

HQ-R4 is elected as DR, and HQ-R3 is elected as BDR.
In OSPF DR/BDR elections on broadcast multiaccess networks, routers first evaluate interface priority values. An interface priority of 0 immediately excludes a router (HQ-R1) from election eligibility. Between HQ-R3 and HQ-R4, both share the highest priority (20). The tie is broken by comparing their Router IDs (derived from active loopback interfaces). Because 2.2.2.22.2.2.2 is greater than 1.1.1.11.1.1.1, HQ-R4 wins the DR election, and HQ-R3 becomes the BDR.

Adım Adım Çözüm

1
Evaluate interface priority eligibility
HQ-R1 has an OSPF priority of 0 and is immediately excluded from becoming DR or BDR, despite having the highest loopback IP address (192.168.100.1192.168.100.1).
An OSPF priority of 0 explicitly configured on an interface renders the router ineligible for DR/BDR election.
2
Compare interface priorities among remaining routers
HQ-R3 and HQ-R4 both have an interface priority of 20, which is strictly higher than HQ-R2's priority of 10.
OSPF DR/BDR election primary criterion is highest interface priority.
3
Break priority tie using Router ID
HQ-R3 has Router ID 1.1.1.11.1.1.1 (from Loopback0) and HQ-R4 has Router ID 2.2.2.22.2.2.2 (from Loopback0). Since 2.2.2.2>1.1.1.12.2.2.2 > 1.1.1.1, HQ-R4 is elected DR, and HQ-R3 is elected BDR.
When interface priorities tie, the highest OSPF Router ID breaks the tie.

Anahtar Kavram

OSPFv2 DR/BDR election precedence (Priority > Router ID) and Priority 0 exclusion
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OSPFv2 Network Types and DR/BDR Selection

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