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Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

A Cisco router is booted with the following interfaces configured and in the up/up state:

- Loopback0: 10.10.10.1/32
- Loopback1: 172.16.100.1/32
- GigabitEthernet0/0: 192.168.10.1/24
- GigabitEthernet0/1: 192.168.200.1/24

An administrator configures OSPFv2 on the router using the following commands:

text
Router(config)# router ospf 1
Router(config-router)# network 192.168.10.0 0.0.0.255 area 0

No `router-id` command is specified. Which IP address will OSPFv2 select as the Router ID for process 1?

  1. 172.16.100.1Answer
  2. B
    192.168.200.1
  3. C
    192.168.10.1
  4. D
    10.10.10.1

Answer

172.16.100.1 is selected because it is the highest IPv4 address among all active loopback interfaces on the router.
OSPFv2 uses a strict order of precedence to determine the Router ID: 1) Explicitly configured `router-id` command, 2) Highest IPv4 address of any active (up/up) loopback interface, 3) Highest IPv4 address of any active non-loopback interface. Since no `router-id` command is configured, OSPF inspects the active loopback interfaces (10.10.10.1 and 172.16.100.1) and selects the highest IP address (172.16.100.1). It is not required for OSPF to be enabled on that loopback interface.

Step-by-Step Solution

1
Evaluate manual Router ID configuration
No explicit `router-id` command was executed under the OSPF process.
Explicit configuration always takes first priority in OSPF Router ID election.
2
Evaluate active loopback interfaces
Two active loopbacks exist: Loopback0 (10.10.10.1) and Loopback1 (172.16.100.1).
If no manual Router ID exists, OSPF evaluates active loopback interfaces before non-loopback physical interfaces.
3
Select the highest loopback IP address
172.16.100.1 is numerically higher than 10.10.10.1.
OSPF selects the highest IPv4 address among active loopback interfaces regardless of whether OSPF is enabled on those specific loopbacks.

Key Concept

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

OSPFv2 Neighbor Adjacencies and Router ID

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