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Zorluk: KolayOSPFv2 Neighbor Adjacencies and Router ID

A network administrator enables OSPFv2 on router R3 without manually configuring an OSPF router ID. The router has the following operational interfaces:

- Loopback0: 10.0.0.1/3210.0.0.1/32
- Loopback1: 172.16.1.1/32172.16.1.1/32
- GigabitEthernet0/0: 192.168.10.1/24192.168.10.1/24
- GigabitEthernet0/1: 192.168.20.1/24192.168.20.1/24

What IPv4 address will OSPFv2 select as the router ID for router R3?

Cevap: 172.16.1.1

Cevap

172.16.1.1
OSPFv2 selects its Router ID using a specific three-step priority order: 1) Manually configured router ID using the router-id command, 2) The highest IPv4 address on any active loopback interface, 3) The highest IPv4 address on any active non-loopback (physical) interface. Since no manual ID is specified, OSPF checks the loopback interfaces. Loopback1 (172.16.1.1172.16.1.1) is numerically higher than Loopback0 (10.0.0.110.0.0.1) and takes precedence over physical interfaces (192.168.20.1192.168.20.1).

Adım Adım Çözüm

1
Check for a manually configured router ID.
No explicit router-id command is configured.
Manual configuration takes precedence over all dynamic election rules.
2
Evaluate active loopback interfaces.
Loopback0 has IP address 10.0.0.1 and Loopback1 has IP address 172.16.1.1. Comparing them, 172.16.1.1 is the highest IP address.
When no manual router ID is specified, OSPF automatically selects the highest IPv4 address among all active loopback interfaces, preferring loopbacks over physical interfaces regardless of IP values.

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OSPFv2 Router ID Selection Logic
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OSPFv2 Neighbor Adjacencies and Router ID

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