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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A network engineer initializes OSPFv2 process 10 on router R-HQ. The router interfaces are configured as follows:

- Loopback 0: IP address 10.1.1.1/32 (Status: administratively down, Protocol: down)
- Loopback 1: IP address 192.168.50.1/32 (Status: up, Protocol: up)
- GigabitEthernet 0/0: IP address 172.16.100.254/24 (Status: up, Protocol: up)
- GigabitEthernet 0/1: IP address 10.254.254.254/30 (Status: up, Protocol: up)

No manual router ID is configured under `router ospf 10`. When the OSPF process starts, which IP address will be selected as the OSPF Router ID for R-HQ?

  1. 192.168.50.1Answer
  2. B
    10.254.254.254
  3. C
    10.1.1.1
  4. D
    172.16.100.254

Answer

192.168.50.1 will be selected as the OSPF Router ID.
OSPF Router ID selection follows a strict order of precedence: 1) Manually configured 'router-id' under the OSPF process, 2) Highest IPv4 address on an active (up/up) loopback interface, 3) Highest IPv4 address on an active (up/up) physical interface. Because no manual Router ID is specified and Loopback 0 is down, the router selects 192.168.50.1 (Loopback 1), which is the only operational loopback interface.

Step-by-Step Solution

1
Check for a manually configured OSPF Router ID
No explicit router-id command is configured.
Explicit configuration is the highest priority in OSPF Router ID selection.
2
Evaluate active (up/up) loopback interfaces
Loopback 0 is down, leaving Loopback 1 (192.168.50.1) as the only active loopback interface.
Active loopback interfaces take precedence over physical interfaces regardless of numerical value.
3
Select the highest IP address among active loopbacks
192.168.50.1 is selected as the OSPF Router ID.
192.168.50.1 is the highest (and only active) loopback interface IP.

Key Concept

OSPF Router ID Election Logic
Estimated Time:1m 15s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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