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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router initializes an OSPFv2 routing process without an explicitly configured OSPF router-id. The interface configuration and operational states are as follows:

• Loopback0: 172.16.1.10/24 (up/up)
• Loopback1: 172.16.2.5/24 (up/up)
• GigabitEthernet0/0: 10.1.1.1/24 (up/up)
• GigabitEthernet0/1: 192.168.100.1/24 (up/up)

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

Answer: 172.16.2.5

Answer

172.16.2.5
OSPFv2 determines the router ID using a strict three-tier priority order: 1) Explicitly configured router-id command under the OSPF process, 2) Highest IPv4 address among operational (up/up) loopback interfaces, 3) Highest IPv4 address among operational physical (non-loopback) interfaces. Because no manual router ID is specified, OSPF evaluates the loopback interfaces. Between Loopback0 (172.16.1.10) and Loopback1 (172.16.2.5), 172.16.2.5 is numerically higher and is selected as the router ID, overriding the physical interface address 192.168.100.1.

Step-by-Step Solution

1
Check for a manually configured OSPF router ID under the OSPF process configuration.
No manual router ID is configured.
Explicit configuration always takes top priority in OSPF router ID selection.
2
Evaluate operational (up/up) loopback interfaces for the highest IPv4 address.
Loopback0 (172.16.1.10) and Loopback1 (172.16.2.5) are active. 172.16.2.5 is numerically greater than 172.16.1.10.
If no manual router ID exists, OSPF selects the highest IPv4 address among active loopback interfaces.
3
Determine the final OSPFv2 router ID without considering physical interfaces.
172.16.2.5 is chosen as the router ID.
Active physical interface IP addresses (such as 192.168.100.1) are only evaluated if zero active loopback interfaces exist.

Key Concept

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

OSPFv2 Neighbor Adjacencies and Router ID

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