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Difficulty: HardOSPFv2 Neighbor Adjacencies and Router ID

An engineer initializes OSPFv2 process 10 on a Cisco router. At initialization, the router has active interfaces Loopback0 (IP address 10.10.10.10/3210.10.10.10/32) and GigabitEthernet0/0 (IP address 192.168.1.1/24192.168.1.1/24). No manual router ID is configured initially. Shortly after OSPF forms adjacencies, the engineer configures interface Loopback1 with IP address 172.16.50.1/32172.16.50.1/32 (up/up) and then executes the `router-id 1.1.1.1` command under the `router ospf 10` configuration prompt. Assuming the command `clear ip ospf process` has NOT been executed and the router has NOT been reloaded, what IP address is currently used as the active OSPF router ID?

Answer: 10.10.10.10

Answer

10.10.10.10
The active OSPF Router ID remains 10.10.10.10 because OSPF Router ID selection is non-preemptive. Upon initial process startup without a manual router ID, the router selected the highest IP address among operational loopback interfaces (10.10.10.10). Subsequent configuration changes—such as adding a loopback with a higher IP address or configuring the `router-id 1.1.1.1` command—do not alter the running OSPF Router ID until the OSPF process is restarted with `clear ip ospf process` or the router is reloaded.

Step-by-Step Solution

1
Determine initial OSPF Router ID election
Since no manual router ID was configured at initialization, OSPF selected the highest IP address among active loopback interfaces, which was 10.10.10.10.
OSPF election order evaluates explicit manual router-id configuration first, followed by active loopback interface IPs, and finally active physical interface IPs.
2
Evaluate the effect of adding a new loopback interface
Adding Loopback1 with IP 172.16.50.1 does not alter the active OSPF Router ID.
OSPF Router ID selection is non-preemptive; once an OSPF process selects a router ID, it does not change dynamically when new interfaces come online.
3
Evaluate the effect of executing the manual `router-id 1.1.1.1` command without restarting the OSPF process
The active OSPF Router ID remains 10.10.10.10.
In Cisco IOS/IOS-XE, manually modifying the OSPF router ID takes effect only after the OSPF process is reset using `clear ip ospf process` or the router is reloaded.

Key Concept

OSPFv2 Router ID Non-Preemptive Election and Modification Behavior
Estimated Time:2m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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