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Difficulty: Very hardOSPFv2 Neighbor Adjacencies and Router ID

An engineer is troubleshooting OSPFv2 neighbor establishment between Router-Edge1 and Router-Edge2 over their directly connected GigabitEthernet0/0/1 link. The running configurations for both routers are provided below:

text
[Router-Edge1]
interface Loopback0
ip address 10.255.255.1 255.255.255.255
shutdown
!
interface Loopback1
ip address 172.16.10.1 255.255.255.255
!
interface GigabitEthernet0/0/1
ip address 192.168.12.1 255.255.255.252
ip mtu 1500
ip ospf hello-interval 10
ip ospf dead-interval 40
ip ospf 1 area 0
!
router ospf 1
router-id 1.1.1.1

[Router-Edge2]
interface Loopback0
ip address 10.255.255.2 255.255.255.255
!
interface GigabitEthernet0/0/1
ip address 192.168.12.2 255.255.255.252
ip mtu 1400
ip ospf hello-interval 10
ip ospf dead-interval 30
ip ospf 1 area 0
!
router ospf 1

Based on these configurations, which TWO statements accurately describe the OSPFv2 neighbor behavior and Router ID operation between these routers? (Select TWO.)

  1. The routers will fail to establish an initial 2-WAY neighbor relationship because the configured OSPF Dead intervals on GigabitEthernet0/0/1 do not match.Answer
  2. If the Dead interval mismatch is corrected, the routers will become stuck in the EXSTART/EXCHANGE state due to the IP MTU mismatch on the GigabitEthernet0/0/1 interfaces.Answer
  3. C
    Router-Edge1 dynamically elects 172.16.10.1 as its Router ID because Loopback0 is administrative shut down.
  4. D
    Router-Edge2 elects 192.168.12.2 as its Router ID because active physical interface IP addresses override loopback interface IP addresses during election.

Answer

The correct statements are that the routers will fail to establish a 2-WAY neighbor state due to mismatched Dead intervals, and if resolved, will remain stuck in EXSTART/EXCHANGE due to the IP MTU mismatch.
The option stating that mismatched Dead intervals prevent the 2-WAY neighbor relationship is correct because Hello and Dead intervals are mandatory parameters in OSPF Hello packets that must match between neighbors. The option noting that an IP MTU mismatch causes routers to become stuck in EXSTART/EXCHANGE is also correct because MTU validation occurs during DBD packet exchanges.

Step-by-Step Solution

1
Evaluate OSPF Hello packet parameter requirements for 2-WAY adjacency formation.
Router-Edge1 is configured with a Dead interval of 40 seconds, whereas Router-Edge2 is configured with 30 seconds. Hello/Dead intervals MUST match exactly between neighbors on a common link. Therefore, Hello packets are rejected, preventing 2-WAY state.
Matching Hello/Dead timers are a mandatory prerequisite for OSPF neighbor discovery and adjacency.
2
Analyze the impact of MTU settings during Database Description (DBD) packet negotiation.
Router-Edge1 has an IP MTU of 1500 bytes, while Router-Edge2 has an IP MTU of 1400 bytes. When MTUs mismatch, routers fail to process DBD packets during Master/Slave negotiation, remaining stuck in EXSTART/EXCHANGE state.
OSPF includes interface MTU in DBD packets to ensure MTU parity across adjacent neighbors.
3
Verify Router ID election logic for Router-Edge1 and Router-Edge2.
Router-Edge1 uses explicit 'router-id 1.1.1.1', which overrides all loopbacks and physical interfaces. Router-Edge2 has no explicit router-id command, so it selects its active Loopback0 IP (10.255.255.2), because loopbacks always take priority over physical interfaces.
Election order is strictly: 1. Manual 'router-id' command -> 2. Highest active loopback IP -> 3. Highest active non-loopback interface IP.

Key Concept

OSPFv2 Adjacency Requirements and Router ID Selection Order
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Topics

OSPFv2 Neighbor Adjacencies and Router ID

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