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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router initializes an OSPFv2 routing process without an explicitly configured router ID. The router currently has the following operational interfaces and statuses:

- Loopback 0: 10.254.254.1/24 (Administratively Down / Down)
- Loopback 1: 172.16.40.1/24 (Up / Up)
- Loopback 2: 172.16.100.1/24 (Up / Up)
- GigabitEthernet0/0: 192.168.10.1/24 (Up / Up)

What IP address will be selected as the OSPFv2 router ID for this router?

Answer: 172.16.100.1

Answer

172.16.100.1
When no manual router ID is configured, OSPFv2 selects the highest IPv4 address among active (up/up) loopback interfaces. Loopback 0 is administratively down, so it is ignored. Between the operational loopbacks (172.16.40.1 and 172.16.100.1), 172.16.100.1 is the highest IP address.

Step-by-Step Solution

1
Check for an explicitly configured router ID under the OSPF process.
No manual 'router-id' command is configured.
Explicit configuration takes highest priority in OSPF router ID election.
2
Evaluate active (up/up) loopback interfaces.
Loopback 1 (172.16.40.1) and Loopback 2 (172.16.100.1) are active. Loopback 0 (10.254.254.1) is administratively down and ignored.
Inactive interfaces cannot be used for router ID election, and active loopbacks take priority over physical interfaces.
3
Select the highest IP address among the active loopback interfaces.
172.16.100.1 is selected because 172.16.100.1 is greater than 172.16.40.1.
OSPF selects the numerically highest IPv4 address among operational loopback interfaces.

Key Concept

OSPFv2 Router ID Election Order
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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