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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router initializes OSPFv2 process 100 with no explicitly configured router ID and no active loopback interfaces. The router has three physical interfaces configured with the following IPv4 addresses and operational states:

- GigabitEthernet0/0: 10.200.15.1/24 (Status: up, Line Protocol: up)
- GigabitEthernet0/1: 172.25.1.254/16 (Status: up, Line Protocol: up)
- GigabitEthernet0/2: 192.168.50.5/30 (Status: up, Line Protocol: down)

Which IPv4 address will OSPFv2 select as the Router ID when the routing process starts?

Answer: 172.25.1.254 / 172.25.1.254/16

Answer

172.25.1.254
When OSPF process initializes without an explicit 'router-id' command or any active loopback interfaces, it chooses the highest IPv4 address among operationally active (up/up) physical interfaces. Since GigabitEthernet0/2 is inactive (up/down), only GigabitEthernet0/0 (10.200.15.1) and GigabitEthernet0/1 (172.25.1.254) are eligible. 172.25.1.254 is the highest active address.

Step-by-Step Solution

1
Evaluate the OSPF Router ID election precedence rules
OSPF selects the router ID using the following priority: 1) Manually configured 'router-id' command, 2) Highest IPv4 address of any active loopback interface, 3) Highest IPv4 address of any active non-loopback physical interface.
Since no manual router ID or loopback interface exists, the selection defaults to active physical interfaces.
2
Filter interfaces by operational status
GigabitEthernet0/0 (10.200.15.1) and GigabitEthernet0/1 (172.25.1.254) are both up/up (active). GigabitEthernet0/2 (192.168.50.5) is up/down (inactive) and is disqualified from router ID selection.
OSPF only considers interfaces whose line protocol is operationally active (up/up).
3
Determine the highest active IPv4 address
Compare active addresses 10.200.15.1 and 172.25.1.254. 172.25.1.254 is numerically higher.
Numeric comparison of active physical interface IPv4 addresses yields 172.25.1.254 as the highest value.

Key Concept

OSPFv2 Router ID election logic prioritizes active loopback interfaces over physical interfaces, selecting the highest IPv4 address among operationally active (up/up) interfaces when no manual router ID is specified.
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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