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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router has OSPFv2 process 1 initialized without an explicitly configured router ID. The interface operational statuses and IPv4 address assignments are as follows:

- Loopback0: 172.25.1.1/32 (administratively down, down/down)
- Loopback1: 10.120.5.1/32 (up/up)
- Loopback2: 10.80.99.1/32 (up/up)
- GigabitEthernet0/0: 192.168.50.1/24 (up/up)

What IPv4 address will OSPFv2 select as the router ID for process 1?

Answer: 10.120.5.1

Answer

10.120.5.1
OSPF selects its Router ID using the following strict hierarchy: 1) Explicitly configured `router-id` command, 2) Highest IPv4 address among active (up/up) loopback interfaces, 3) Highest IPv4 address among active (up/up) physical interfaces. Here, Loopback1 (10.120.5.1) and Loopback2 (10.80.99.1) are both active loopbacks. Since 10.120.5.1 is numerically higher than 10.80.99.1, 10.120.5.1 is chosen.

Step-by-Step Solution

1
Check for a manually configured router ID
No explicit `router-id` command was configured under the OSPF process.
Manual configuration always takes highest precedence in OSPF router ID election.
2
Evaluate operational loopback interfaces
Loopback1 (10.120.5.1) and Loopback2 (10.80.99.1) are both active (up/up). Loopback0 is down/down and excluded.
Only operational (up/up) interfaces are eligible for automatic election. Active loopback interfaces take precedence over physical interfaces.
3
Compare IP addresses of active loopbacks to select the highest numerical IP
Comparing 10.120.5.1 and 10.80.99.1, 10.120.5.1 is numerically higher because 120>80120 > 80 in the second octet.
OSPF selects the highest IPv4 address among all operational loopback interfaces.

Key Concept

OSPFv2 Router ID Election Logic
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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