A network engineer starts an OSPFv2 routing process on router Branch-GW. Prior to launchin… — Cisco CCNA

A network engineer starts an OSPFv2 routing process on router Branch-GW. Prior to launching the OSPF process, no explicit router-id command is configured. The router has the following interfaces configured:

- Interface Loopback0: IP address $10.200.1.1/32$ , Operational Status: Administratively Down
- Interface Loopback10: IP address $172.16.50.1/24$ , Operational Status: Up/Up
- Interface Loopback20: IP address $172.16.20.1/24$ , Operational Status: Up/Up
- Interface GigabitEthernet0/0: IP address $192.168.10.1/24$ , Operational Status: Up/Up

What IPv4 address will OSPFv2 select as the Router ID for Branch-GW?

Answer: 172.16.50.1

Answer

172.16.50.1

In Cisco OSPFv2, the Router ID selection order follows three rules: 1) Explicitly configured `router-id` command, 2) Highest IPv4 address among active (Up/Up) loopback interfaces, 3) Highest IPv4 address among active (Up/Up) physical interfaces. Since no manual Router ID is configured, OSPF evaluates loopback interfaces. Loopback0 is administratively down, so it is ineligible. Between the active loopbacks (Loopback10 at 172.16.50.1 and Loopback20 at 172.16.20.1), 172.16.50.1 is the highest IP address, making it the selected Router ID.

Step-by-Step Solution

Check for a manually configured OSPF Router ID

No explicit OSPF router-id command was entered under the routing process.

Explicit configuration always takes top precedence in OSPF Router ID election.

Evaluate active (Up/Up) Loopback interfaces

Loopback0 (

10.200.1.1

) is administratively down and ignored. Active loopbacks are Loopback10 (

172.16.50.1

) and Loopback20 (

172.16.20.1

Only active interfaces with operational status Up/Up are eligible for Router ID selection.

Select the highest IPv4 address among eligible active loopback interfaces

Comparing

172.16.50.1

and

172.16.20.1

, the address

172.16.50.1

is numerically higher.

OSPF tie-breaking logic favors the highest IPv4 address among active loopback interfaces over physical interface IP addresses.

Key Concept

OSPFv2 Router ID Election Order and Interface State Requirements

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

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