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Zorluk: Çok zorOSPFv2 Neighbor Adjacencies and Router ID

A network administrator initializes OSPFv2 on Router R1 when only interface GigabitEthernet0/0 (10.0.12.1/24) and Loopback 1 (172.31.1.1/24) are active. Later, the administrator performs two additional configuration changes on R1 without restarting the OSPF process:
1. Brings up Loopback 0 with IP address 10.254.254.1/32.
2. Configures `router-id 192.168.1.1` under the OSPF routing process.

Directly connected to R1's GigabitEthernet0/0 interface is Router R2 (10.0.12.2/24). R1's GigabitEthernet0/0 interface is assigned to OSPF Area 0, whereas R2's GigabitEthernet0/0 interface is configured for OSPF Area 1. Both routers share identical Hello/Dead timers and MTU settings.

Assuming no OSPF process resets have occurred, what is R1's active OSPF Router ID, and what neighbor relationship status will R1 report for R2?

  1. R1's active Router ID is 172.31.1.1, and no OSPF neighbor entry is established for R2.Cevap
  2. B
    R1's active Router ID is 192.168.1.1, and the neighbor state is stuck in INIT.
  3. C
    R1's active Router ID is 10.254.254.1, and the neighbor state is stuck in 2-WAY.
  4. D
    R1's active Router ID is 172.31.1.1, and the neighbor state is stuck in EXSTART.

Cevap

R1's active Router ID remains 172.31.1.1, and no OSPF neighbor relationship is formed (no neighbor entry appears in R1's OSPF neighbor table).
OSPF Router ID election occurs when the OSPF process initializes. Once an active Router ID is chosen (in this case, 172.31.1.1 from the highest active loopback at process start), subsequent configuration changes—such as setting a manual 'router-id' or activating another loopback—do not alter the active Router ID until the OSPF process is explicitly reset via 'clear ip ospf process'. Furthermore, OSPF neighbor discovery requires matching Area IDs in Hello packets. Because R1 is in Area 0 and R2 is in Area 1, incoming Hello packets are dropped, resulting in no OSPF neighbor relationship being formed at all.

Adım Adım Çözüm

1
Determine R1's active OSPF Router ID.
R1 selected 172.31.1.1 upon OSPF initialization because Loopback 1 was the highest IP among active loopback interfaces.
OSPF Router ID election logic follows: manual 'router-id' > highest active loopback IP > highest active physical interface IP. However, Router ID election is non-preemptive. Adding a manual 'router-id' or bringing up a higher loopback IP (Loopback 0) later does NOT update the active Router ID until the process is restarted using 'clear ip ospf process' or a system reboot.
2
Evaluate the OSPF Hello packet exchange and neighbor adjacency conditions.
R1 (Area 0) and R2 (Area 1) have an Area ID mismatch in their OSPF Hello packet headers.
For two OSPFv2 routers to establish a neighbor relationship, key parameters in the Hello packet must match identically: Area ID, Subnet Mask, Hello/Dead Timers, Authentication, and Stub Area flag. If the Area ID mismatches, received Hello packets are silently discarded.
3
Determine the resulting OSPF neighbor state.
No neighbor state (or neighbor entry) exists.
Because Hello packets are dropped on receipt due to the Area ID mismatch, neither router discovers the other, preventing even the INIT state from being reached.

Anahtar Kavram

OSPFv2 Router ID Stability & Mandatory Hello Packet Adjacency Parameters
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Konular

OSPFv2 Neighbor Adjacencies and Router ID

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