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Difficulty: Very hardOSPFv2 Neighbor Adjacencies and Router ID

An administrator is troubleshooting OSPFv2 neighbor connectivity between Router A and Router B connected directly over a GigabitEthernet broadcast segment. Both routers are configured in OSPF Area 0.

Router A has the following operational status:
- Loopback0 interface IP: 172.16.1.1/24 (Up/Up)
- Loopback1 interface IP: 192.168.100.1/24 (Up/Up)
- GigabitEthernet0/1 interface IP: 10.1.12.1/24 (Up/Up)
- Ten minutes after the OSPF process initialized, the administrator executed `router-id 1.1.1.1` under `router ospf 1`, but did not restart the OSPF process.
- An inbound IPv4 ACL applied to GigabitEthernet0/1 permits IP unicast traffic from Router B, but drops all IP multicast traffic addressed to 224.0.0.5.

Router B has the following operational status:
- Loopback0 interface IP: 10.2.2.2/32 (Up/Up)
- GigabitEthernet0/1 interface IP: 10.1.12.2/24 (Up/Up)
- Active OSPF Router ID: 10.2.2.2

When executing `show ip ospf neighbor` on Router B, the neighbor state for Router A is listed as `INIT/ -`. However, executing `show ip ospf neighbor` on Router A displays no output.

Which statement accurately explains why Router B is in the INIT state while Router A shows no neighbor, and identifies the Router ID that Router B receives from Router A?

  1. Router B receives outgoing multicast Hellos from Router A containing Router ID 192.168.100.1 and moves Router A into the INIT state; Router A displays no neighbor because its inbound ACL drops Router B's multicast Hello packets.Answer
  2. B
    Router B receives outgoing multicast Hellos from Router A containing Router ID 1.1.1.1 because the router-id command updates the active Router ID immediately, but Router A remains in DOWN state due to an Area ID mismatch.
  3. C
    Router B receives outgoing multicast Hellos from Router A containing Router ID 172.16.1.1 because Loopback0 is preferred over Loopback1, while Router A shows no neighbor because both routers have a DR priority of 0.
  4. D
    Router B receives outgoing multicast Hellos from Router A containing Router ID 10.1.12.1 because physical interfaces supersede loopback interfaces, but Router A fails to recognize Router B due to mismatched Hello timers.

Answer

Router B receives outgoing multicast Hellos from Router A containing Router ID 192.168.100.1 and moves Router A into the INIT state; Router A displays no neighbor because its inbound ACL drops Router B's multicast Hello packets.
The correct statement identifies that Router A's active Router ID is 192.168.100.1 (the highest active loopback IP address at startup, because the `router-id` command is non-preemptive until the OSPF process is cleared). Router B receives Router A's multicast Hello packets containing this Router ID and transitions Router A to the INIT state because Router B is not yet listed in Router A's Hello neighbor field. Meanwhile, Router A receives zero Hello packets from Router B because its inbound ACL filters all multicast traffic to 224.0.0.5, causing Router A to remain in the DOWN state with no neighbor table entries.

Step-by-Step Solution

1
Determine Router A's active OSPF Router ID
Router A's active Router ID is 192.168.100.1.
When OSPF initializes without an explicit command, the router selects the highest IP address among active loopback interfaces. Loopback1 (192.168.100.1) is higher than Loopback0 (172.16.1.1). The `router-id 1.1.1.1` command was entered later but is non-preemptive; it requires `clear ip ospf process` or a reload to take effect.
2
Analyze outbound Hello packet transmission from Router A to Router B
Router B receives Router A's Hello packet containing Router ID 192.168.100.1.
Router A sends multicast Hello packets to 224.0.0.5 out GigabitEthernet0/1. Since Router A hasn't received Hellos from Router B yet, Router B's Router ID is not listed in the neighbor field of Router A's Hello packet.
3
Analyze Router B's state transition upon receiving Router A's Hello
Router B transitions Router A to the INIT state.
When a router receives a valid Hello packet from a neighbor that does not yet list the receiving router's own Router ID, it places that neighbor into the INIT state.
4
Analyze inbound packet processing on Router A
Router A drops Router B's multicast Hello packets (224.0.0.5) and shows no neighbor entry.
Router B responds by sending multicast Hellos to 224.0.0.5 containing Router A's Router ID in its neighbor list. However, Router A's inbound ACL drops all traffic to 224.0.0.5. As a result, Router A never receives Router B's Hellos, remains in the DOWN state, and displays no output for `show ip ospf neighbor`.

Key Concept

OSPFv2 Router ID election precedence (manual command requiring process reset vs. highest active loopback) combined with unidirectional Hello packet flow causing one-sided INIT neighbor states.
Estimated Time:3m 0s
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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