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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network administrator initializes an OSPFv2 routing process on a Cisco router without explicitly configuring an OSPF router ID under the router configuration mode. The router displays the following interface status and IP address assignments:

- Loopback0: IP address 10.10.10.1/32, status up, line protocol up
- Loopback1: IP address 192.168.1.1/32, status administratively down, line protocol down
- GigabitEthernet0/0: IP address 172.16.1.1/24, status up, line protocol up
- GigabitEthernet0/1: IP address 192.168.100.1/24, status up, line protocol up

Which IP address will OSPFv2 select as the router ID when the routing process starts?

  1. 10.10.10.1Answer
  2. B
    192.168.1.1
  3. C
    192.168.100.1
  4. D
    172.16.1.1

Answer

The router will select 10.10.10.1 (Loopback0) as the OSPFv2 Router ID.
OSPFv2 determines its Router ID using a strict three-tier precedence: (1) manual configuration via the 'router-id' command, (2) the highest IPv4 address among active (up/up) loopback interfaces, and (3) the highest IPv4 address among active non-loopback physical interfaces. Because Loopback0 (10.10.10.1) is operational (up/up) and Loopback1 is administratively down, Loopback0 provides the highest active loopback IP address, overriding all physical interface IPs.

Step-by-Step Solution

1
Check for manual configuration
No explicit 'router-id' command was configured under the OSPF process.
Manual configuration always takes highest precedence.
2
Evaluate loopback interfaces
Loopback0 (10.10.10.1) is up/up. Loopback1 (192.168.1.1) is administratively down.
Only active (up/up) interfaces are considered for router ID election.
3
Apply tie-breaking rules for loopback vs physical interfaces
Loopback0 is the only active loopback interface, so its IP address 10.10.10.1 is selected as the Router ID.
Active loopback interfaces are preferred over active physical interfaces regardless of numerical IP address values.

Key Concept

OSPFv2 Router ID election precedence logic
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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