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Difficulty: EasyOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router starts an OSPFv2 process without an explicitly configured router ID using the 'router-id' command. The router has the following active interfaces:

- Loopback 0: 10.1.1.1/32
- Loopback 1: 10.2.2.2/32
- GigabitEthernet 0/0: 172.16.10.1/24
- GigabitEthernet 0/1: 192.168.1.1/24

Which IPv4 address will OSPFv2 select as the router ID?

  1. 10.2.2.2Answer
  2. B
    192.168.1.1
  3. C
    10.1.1.1
  4. D
    172.16.10.1

Answer

10.2.2.2
When no manual router ID is specified via the 'router-id' command, OSPFv2 selects the highest IPv4 address of any active loopback interface. Between Loopback 0 (10.1.1.1) and Loopback 1 (10.2.2.2), 10.2.2.2 is the higher IP address and is selected as the Router ID.

Step-by-Step Solution

1
Check for an explicitly configured router ID
No explicit 'router-id' command has been configured.
Explicit configuration takes top priority in OSPF router ID selection.
2
Evaluate active loopback interfaces
Two active loopbacks exist: 10.1.1.1 and 10.2.2.2.
Loopback interfaces take precedence over physical interfaces.
3
Select the highest IPv4 address among active loopback interfaces
10.2.2.2 is higher than 10.1.1.1, making 10.2.2.2 the chosen OSPF Router ID.
When multiple loopbacks are present, OSPF breaks the tie using the highest IPv4 address.

Key Concept

OSPFv2 Router ID Selection Priority
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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