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Difficulty: EasyOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router initializes an OSPFv2 process without a manually configured router ID. The router currently has the following operational interfaces:

- Loopback0: 10.10.10.1/3210.10.10.1/32
- Loopback10: 10.20.20.1/3210.20.20.1/32
- GigabitEthernet0/1: 192.168.100.1/24192.168.100.1/24

What IP address will OSPFv2 automatically select as the router ID?

Answer: 10.20.20.1

Answer

10.20.20.1
When an explicit OSPF router ID is not configured using the 'router-id' command, OSPFv2 selects the highest IPv4 address among all active loopback interfaces. Because loopback interfaces take priority over physical interfaces, the router compares 10.10.10.1 and 10.20.20.1, choosing 10.20.20.1 as the router ID.

Step-by-Step Solution

1
Check if a manual OSPF router ID is configured.
No manual router ID is specified under the OSPF process.
Explicit configuration taking highest precedence is absent.
2
Evaluate active loopback interfaces.
Two active loopback interfaces exist: Loopback0 (10.10.10.110.10.10.1) and Loopback10 (10.20.20.110.20.20.1).
OSPF criteria prioritize active loopback interface IP addresses over active physical interface IP addresses.
3
Determine the highest IP address among active loopback interfaces.
10.20.20.110.20.20.1 is numerically higher than 10.10.10.110.10.10.1.
10.20.20.110.20.20.1 is chosen as the router ID.

Key Concept

OSPFv2 Router ID Election Order
Estimated Time:45s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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