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Zorluk: OrtaOSPFv2 Network Types and DR/BDR Selection

An engineer executes the command shown below to inspect the OSPFv2 operational status of interface GigabitEthernet0/1 on router R3:

text
R3# show ip ospf interface GigabitEthernet0/1
GigabitEthernet0/1 is up, line protocol is up
Internet Address 10.1.1.3/24, Area 0
Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DROTHER, Priority 0
Designated Router (ID) 2.2.2.2, Interface address 10.1.1.2
Backup Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Why does router R3 operate in the DROTHER state despite having a higher Router ID than both Designated Router R2 (2.2.2.2) and Backup Designated Router R1 (1.1.1.1)?

  1. The interface priority on R3 is set to 0, which completely excludes the router from participating in DR and BDR elections.Cevap
  2. B
    The higher Router ID of 3.3.3.3 is overridden because R3 joined the multiaccess segment after R1 and R2 were elected as DR and BDR.
  3. C
    An interface priority of 0 allows a router to participate only in Backup Designated Router (BDR) elections, leaving the Designated Router role to R2.
  4. D
    R3 remains a DROTHER because its configured Hello timer interval of 10 seconds does not match the DR timer settings.

Cevap

The interface priority on R3 is set to 0, which completely excludes the router from participating in DR and BDR elections.
In OSPFv2 multiaccess networks, DR and BDR elections evaluate interface priority first (highest priority wins, default is 1). Configuring an interface priority of 0 explicitly prevents that router interface from taking on either the DR or BDR role, keeping it permanently as a DROTHER regardless of how high its Router ID is.

Adım Adım Çözüm

1
Examine the output of the 'show ip ospf interface GigabitEthernet0/1' command on router R3.
The command output confirms 'State DROTHER, Priority 0' alongside Router ID 3.3.3.3, DR ID 2.2.2.2, and BDR ID 1.1.1.1.
This reveals R3's current operational state, its configured priority, and the current election roles on the segment.
2
Evaluate the impact of interface priority on OSPF DR/BDR election eligibility.
An OSPF interface priority of 0 explicitly disqualifies a router from participating in DR and BDR elections.
By rule, any router with a priority of 0 is immediately excluded from election candidacy regardless of its Router ID value.
3
Determine why R3 remains in the DROTHER state.
Because R3's priority is set to 0, its higher Router ID (3.3.3.3) is ignored during elections, placing R3 in the DROTHER state.
Router ID serves as a tie-breaker only among routers that have non-zero priority values.

Anahtar Kavram

OSPFv2 DR/BDR Election Rules and Priority 0 Exclusion
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OSPFv2 Network Types and DR/BDR Selection

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