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Zorluk: OrtaOSPFv2 Neighbor Adjacencies and Router ID

A network administrator configures OSPFv2 process 1 on router R4 without manually specifying an OSPF router ID. Prior to starting the OSPF process, R4 has the following operational interface statuses and IPv4 addresses:

- Loopback 0: 10.200.1.1/24 (Administratively Down, Line Protocol Down)
- Loopback 1: 172.16.5.1/24 (Up, Line Protocol Up)
- Loopback 2: 172.16.20.1/24 (Up, Line Protocol Up)
- GigabitEthernet 0/0: 192.168.1.254/24 (Up, Line Protocol Up)

What IPv4 address will R4 select as its OSPF Router ID?

Cevap: 172.16.20.1

Cevap

172.16.20.1
When OSPFv2 initializes without a manually configured router ID, it selects the highest IPv4 address among operational (up/up) loopback interfaces. Loopback 0 is ignored because it is administratively down. Between the remaining active loopback interfaces, 172.16.20.1 on Loopback 2 is higher than 172.16.5.1 on Loopback 1. Active loopbacks take absolute precedence over physical interfaces, so GigabitEthernet 0/0 is not evaluated.

Adım Adım Çözüm

1
Check for manual router-id configuration
No explicit router-id command is configured under OSPFv2 process 1.
Manual configuration is the highest priority in OSPF Router ID selection.
2
Identify eligible operational loopback interfaces
Loopback 0 is excluded because its interface state is administratively down. Loopback 1 (172.16.5.1) and Loopback 2 (172.16.20.1) are eligible because both are Up/Up.
OSPF election logic considers active (up/up) loopback interfaces next, prioritizing them over active physical interfaces.
3
Select the highest IP address among eligible loopback interfaces
172.16.20.1 is selected as the Router ID.
Between the two active loopbacks, 172.16.20.1 is numerically higher than 172.16.5.1. The physical interface address 192.168.1.254 is ignored because active loopback interfaces take precedence.

Anahtar Kavram

OSPFv2 Router ID Election Order
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OSPFv2 Neighbor Adjacencies and Router ID

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