A Cisco router has the following interface operational statuses and IPv4 configurations:

- GigabitEthernet0/0 ( $192.168.50.1/24$ ): line protocol up, operational state UP
- GigabitEthernet0/1 ( $192.168.100.1/24$ ): line protocol up, operational state UP
- Loopback 0 ( $10.254.1.1/32$ ): administratively down, line protocol down
- Loopback 1 ( $10.200.1.1/32$ ): line protocol down, operational state DOWN
- Loopback 2 ( $172.16.20.1/32$ ): line protocol up, operational state UP

The OSPFv2 routing process is initialized using `router ospf 1` without an explicitly configured static Router ID via the `router-id` command. What exact IPv4 address will OSPFv2 elect as the active Router ID for this process?

Answer: 172.16.20.1

Answer

172.16.20.1

OSPFv2 uses a strict three-tier precedence logic to select the Router ID: 1) An explicitly configured 'router-id' command, 2) The highest IPv4 address among active (up/up) loopback interfaces, 3) The highest IPv4 address among active (up/up) physical non-loopback interfaces. Here, no explicit Router ID exists. Loopback 0 and Loopback 1 are both down, making Loopback 2 (172.16.20.1) the sole active loopback interface. Because active loopback interfaces take precedence over physical interfaces (such as GigabitEthernet0/1 with 192.168.100.1), 172.16.20.1 is selected as the Router ID.

Step-by-Step Solution

Check for an explicitly configured router ID using the 'router-id' command.

No explicit router ID is configured under 'router ospf 1'.

Explicit configuration takes highest priority in OSPF Router ID selection.

Evaluate all active (up/up) loopback interfaces to find the highest IPv4 address.

Loopback 0 and Loopback 1 are inactive (down). Loopback 2 (172.16.20.1) is active (up/up).

OSPF prefers active loopback interfaces over physical interfaces, but inactive loopbacks cannot participate in election.

Compare candidate addresses among active loopbacks.

172.16.20.1 is the only active loopback address.

Since Loopback 2 is the only active loopback interface, its IP address (172.16.20.1) is chosen over the physical interface IP addresses.

Key Concept

OSPFv2 Router ID Selection Priority Logic

Estimated Time:1m 30s

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