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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router has three active interfaces with the following operational statuses and IPv4 addresses:
- Loopback 10: 10.200.1.1/24 (up/up)
- Loopback 20: 172.16.50.1/24 (up/up)
- GigabitEthernet 0/0: 192.168.100.1/24 (up/up)

The OSPFv2 process is initialized using the `router ospf 100` command without an explicitly configured router-id command.

What IPv4 address will be selected as the OSPFv2 Router ID for this router?

Answer: 172.16.50.1

Answer

172.16.50.1
OSPFv2 determines its Router ID using a strict three-tier precedence logic: (1) an explicitly configured OSPF router-id command, (2) the highest IPv4 address among active loopback interfaces, and (3) the highest IPv4 address among active non-loopback (physical) interfaces. Because no manual router ID was configured, the router compares the loopback interfaces (10.200.1.1 and 172.16.50.1) and chooses the highest value, 172.16.50.1. Physical interfaces like GigabitEthernet 0/0 are not considered because active loopbacks exist.

Step-by-Step Solution

1
Check for manual OSPF Router ID configuration
No explicit `router-id` command was configured under the `router ospf 100` process.
Manual configuration always takes highest precedence in OSPF Router ID selection.
2
Evaluate active loopback interfaces
Two active loopback interfaces exist: Loopback 10 (10.200.1.1) and Loopback 20 (172.16.50.1).
In the absence of a manually configured Router ID, OSPF selects the highest IPv4 address among active loopback interfaces.
3
Compare loopback IPv4 addresses
172.16.50.1 is numerically higher than 10.200.1.1, so 172.16.50.1 is chosen.
Physical interface IPs (such as GigabitEthernet 0/0 with 192.168.100.1) are ignored when active loopback interfaces are present.

Key Concept

OSPFv2 Router ID Selection Precedence
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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