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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router is configured to run OSPFv2 without an explicitly set `router-id` under the OSPF process. The current interface IP address configurations and states are as follows:

- Loopback0: 10.10.10.10/3210.10.10.10/32 (Administratively Down)
- Loopback1: 172.16.1.1/24172.16.1.1/24 (Up / Up)
- Loopback2: 192.168.100.1/24192.168.100.1/24 (Up / Up)
- GigabitEthernet0/0: 10.200.200.1/3010.200.200.1/30 (Up / Up)

What IP address will OSPF select as the router ID when the OSPF process initializes?

Answer: 192.168.100.1

Answer

192.168.100.1
OSPFv2 uses a strict order of preference to establish its Router ID: 1) Explicitly configured `router-id` command, 2) Highest IP address among active (Up/Up) loopback interfaces, 3) Highest IP address among active physical interfaces. In this scenario, no manual router ID exists. Loopback0 is down so it is excluded. Loopback1 (172.16.1.1172.16.1.1) and Loopback2 (192.168.100.1192.168.100.1) are both active loopback interfaces. OSPF selects 192.168.100.1192.168.100.1 because it is the highest active loopback IP address.

Step-by-Step Solution

1
Check for an explicitly configured OSPF router ID
No manual OSPF router ID command is configured.
Explicit configuration takes precedence over all automatic election rules.
2
Evaluate all active (Up/Up) loopback interfaces
Loopback1 (172.16.1.1172.16.1.1) and Loopback2 (192.168.100.1192.168.100.1) are active. Loopback0 (10.10.10.1010.10.10.10) is administratively down and ignored.
OSPF election logic prioritizes active loopback interfaces over physical interfaces.
3
Compare IP addresses of active loopbacks to select the highest value
Between 172.16.1.1172.16.1.1 and 192.168.100.1192.168.100.1, 192.168.100.1192.168.100.1 is numerically higher.
OSPF selects the highest IP address among operational loopback interfaces.

Key Concept

OSPFv2 Router ID Selection Precedence
Estimated Time:1m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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