Examkin

Question

Difficulty: Very hardOSPFv2 Neighbor Adjacencies and Router ID

A network engineer is configuring OSPFv2 on router R1, which has the following interface states and IPv4 configurations:

- Loopback0: 192.168.254.1/32 (administratively down, line protocol down)
- Loopback1: 172.16.50.1/24 (up, line protocol up)
- Loopback2: 10.255.255.1/24 (up, line protocol up)
- GigabitEthernet0/0: 10.1.1.1/30 (up, line protocol up)
- GigabitEthernet0/1: 192.168.1.1/30 (up, line protocol up)

The engineer starts the OSPF process using `router ospf 1`. Immediately after, the engineer configures `router-id 1.1.1.1` under the OSPF process prompt but does NOT issue a `clear ip ospf process` command. Router R1 then attempts to establish an OSPFv2 adjacency over GigabitEthernet0/0 (Area 0) with directly connected router R2. Router R2 already has a manually configured static OSPF Router ID of 172.16.50.1172.16.50.1.

Which outcome correctly describes the OSPF neighbor state and behavior between R1 and R2?

  1. The routers fail to establish an OSPF adjacency because both routers are using the duplicate Router ID 172.16.50.1.Answer
  2. B
    The routers successfully reach FULL state because R1 immediately adopted the Router ID 1.1.1.1 upon configuration.
  3. C
    The routers fail to establish an OSPF adjacency because R1 selected 192.168.254.1 from Loopback0, which is unreachable over Area 0.
  4. D
    The routers reach FULL state because R1 dynamically selected 192.168.1.1 as the highest active physical interface IP address.

Answer

The routers fail to establish an OSPF adjacency because both routers are using the duplicate Router ID 172.16.50.1.
When the OSPF process initializes on R1, it evaluates active interfaces. Loopback0 is down, so it is ignored. Loopback1 (172.16.50.1172.16.50.1) is chosen over Loopback2 (10.255.255.110.255.255.1) because it has the higher IPv4 address. Although the administrator configures `router-id 1.1.1.1`, OSPF Router ID changes are non-preemptive and require the process to be cleared (`clear ip ospf process`) before taking effect. Consequently, R1 maintains its elected Router ID of 172.16.50.1172.16.50.1, which matches R2's Router ID. OSPF neighbors cannot form an adjacency with duplicate Router IDs on the same network segment.

Step-by-Step Solution

1
Evaluate initial dynamic OSPF Router ID election on R1
Loopback0 is down and disqualified. Among active loopbacks (Loopback1: 172.16.50.1 and Loopback2: 10.255.255.1), 172.16.50.1 is numerically highest and becomes R1's active Router ID.
OSPF Router ID selection order is: manual 'router-id' command > highest IP on active loopbacks > highest IP on active physical interfaces.
2
Evaluate the effect of the manual 'router-id 1.1.1.1' command
R1 retains 172.16.50.1 as its active Router ID because the process was not cleared.
In Cisco IOS OSPFv2, manually modifying the Router ID on an existing process is non-preemptive until 'clear ip ospf process' is executed.
3
Analyze neighbor adjacency formation with R2
Both R1 and R2 present a Router ID of 172.16.50.1 in Hello packets, leading to duplicate Router ID conflict and adjacency failure.
OSPF routers on the same segment or area require unique Router IDs to establish neighbor relationships.

Key Concept

OSPFv2 Router ID Selection Precedence and Non-Preemptive CLI Behavior
Rate this question

Topics

OSPFv2 Neighbor Adjacencies and Router ID

More questions from this topic