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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

Router R3 is configured with the following interfaces and IP address states:

- Loopback0: 10.255.255.1/32 (Status: up, Line Protocol: up)
- Loopback1: 172.16.100.1/32 (Status: administratively down, Line Protocol: down)
- GigabitEthernet0/0: 192.168.1.1/24 (Status: up, Line Protocol: up)

OSPFv2 process 1 is initialized on R3 without a manually configured router ID. Later, a network engineer enters the following commands:

R3(config)# router ospf 1
R3(config-router)# router-id 1.1.1.1

No additional commands are executed and the OSPF process is not reset. What IPv4 address is currently active as the OSPF router ID for process 1?

Answer: 10.255.255.1

Answer

10.255.255.1
Upon initial process startup, OSPF selects 10.255.255.1 because active loopback interfaces take priority over physical interfaces, and Loopback0 is the only active loopback (Loopback1 is administratively down). When the 'router-id 1.1.1.1' command is subsequently entered under the OSPF process, Cisco IOS requires the process to be reset (e.g., via 'clear ip ospf process') before the change becomes active. Since no reset occurred, 10.255.255.1 remains the active OSPF router ID.

Step-by-Step Solution

1
Determine initial OSPF Router ID election upon process initialization
10.255.255.1 is chosen as the active router ID
When OSPF process 1 starts without a manual router-id command, it selects the highest IPv4 address among active (up/up) loopback interfaces. Loopback0 (10.255.255.1) is active, while Loopback1 is administratively down and ignored. Physical interfaces like GigabitEthernet0/0 are ignored because an active loopback interface exists.
2
Evaluate the impact of configuring 'router-id 1.1.1.1' on a running OSPF process
The new router ID 1.1.1.1 is queued but does not take immediate effect
OSPF Router ID selection in Cisco IOS is non-preemptive. Changing the router ID under the OSPF process does not dynamically update the active router ID for an already running process.
3
Check if any process reload or reset command was executed
No reset occurred; the original router ID remains active
A process restart (such as executing 'clear ip ospf process') or device reload is required to apply the newly configured router ID. Because no reset was performed, 10.255.255.1 remains the active router ID.

Key Concept

OSPFv2 Router ID Election Precedence and Non-Preemptive Re-election
Estimated Time:2m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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