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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network engineer initializes OSPFv2 on a Cisco router with process ID 10. The router interfaces are configured with the following parameters and operational states:

- Loopback 0: 10.10.10.1/32 (administratively down, line protocol down)
- Loopback 1: 172.16.10.1/32 (up, line protocol up)
- GigabitEthernet 0/0: 192.168.10.254/24 (up, line protocol up)
- GigabitEthernet 0/1: 192.168.20.1/24 (up, line protocol up)

No explicit `router-id` command has been configured under the OSPF routing process. Which TWO statements accurately describe the OSPF Router ID selection and operational behavior for this router?

  1. The OSPF process selects 172.16.10.1 as its Router ID because it is the highest IPv4 address among operational loopback interfaces.Answer
  2. The router maintains 172.16.10.1 as its active OSPF Router ID even if Loopback 1 is subsequently shut down, until the OSPF process is reset or reloaded.Answer
  3. C
    The OSPF process selects 192.168.20.1 as its Router ID because GigabitEthernet 0/1 has the highest overall IP address assigned to an active interface.
  4. D
    The OSPF process selects 10.10.10.1 as its Router ID because Loopback 0 is always evaluated first regardless of interface operational status.

Answer

The OSPF process selects 172.16.10.1 as its Router ID because it is the highest IPv4 address among operational loopback interfaces, and it maintains 172.16.10.1 as its active OSPF Router ID even if Loopback 1 is subsequently shut down, until the OSPF process is reset or reloaded.
OSPFv2 selects its Router ID following a strict hierarchy: first an explicit `router-id` configuration, second the highest IPv4 address among operational (up/up) loopback interfaces, and third the highest IPv4 address among operational physical interfaces. Because Loopback 0 is down, Loopback 1 (172.16.10.1) is chosen as the highest active loopback IP address, taking precedence over physical interfaces. Furthermore, OSPF Router ID selection is non-preemptive; once selected, changes to interface states do not immediately update the running Router ID without explicitly resetting the OSPF process.

Step-by-Step Solution

1
Evaluate the order of precedence for OSPFv2 Router ID election.
Order: 1. Manually configured `router-id` under `router ospf`. 2. Highest IPv4 address on an active (up/up) loopback interface. 3. Highest IPv4 address on an active (up/up) physical interface.
Since no manual `router-id` command is present, the process moves to checking operational loopback interfaces.
2
Filter interfaces by operational status (up/up).
Loopback 0 is down and disqualified. Loopback 1 (172.16.10.1), GigabitEthernet 0/0 (192.168.10.254), and GigabitEthernet 0/1 (192.168.20.1) are operational.
Only active interfaces with operational line protocol up can be selected.
3
Determine the winning interface IP address.
Loopback 1 (172.16.10.1) is selected because it is the only active loopback interface, overriding higher IP addresses on physical interfaces.
Loopback interfaces always take precedence over non-loopback interfaces.
4
Analyze non-preemptive behavior of the OSPF Router ID.
Once elected, the Router ID remains stable in memory. Shutting down the interface or adding higher IP addresses does not automatically change the active Router ID until `clear ip ospf process` or a device reload occurs.
OSPF Router ID selection is non-preemptive to prevent unnecessary adjacency tear-downs.

Key Concept

OSPFv2 Router ID Election Priority and Non-Preemptive Stability Mechanics
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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