Examkin

Question

Difficulty: MediumOSPFv2 Neighbor Adjacencies and Router ID

An administrator executes the command `show ip ospf neighbor` on a router and observes that a connected neighbor remains continuously in the INIT state. Which condition is the most likely cause of this issue?

  1. A
    The maximum transmission unit (MTU) size differs between the connected router interfaces.
  2. One router is rejecting incoming Hello packets because of an OSPF Area ID mismatch.Answer
  3. C
    Both routers are configured with an OSPF interface priority of 0 on a broadcast network.
  4. D
    The OSPF process IDs configured on the two routers do not match.

Answer

One router is rejecting incoming Hello packets because of an OSPF Area ID mismatch.
The correct answer identifies that an Area ID mismatch causes incoming Hello packets to be discarded. In OSPF, a router enters the INIT state when it receives a valid Hello packet from a neighbor. However, if the neighbor drops incoming Hellos from the local router due to an Area ID mismatch, the neighbor will never add the local router's Router ID to its Hello packet neighbor field. Consequently, the local router remains stuck in the INIT state.

Step-by-Step Solution

1
Analyze OSPF neighbor state definitions.
The INIT state indicates that a router has received a Hello packet from a neighbor, but the receiving router's own Router ID was not present in that Hello packet.
For OSPF neighbors to transition from INIT to 2-WAY, bidirectional communication must be established by seeing one's own Router ID in the received Hello packet's neighbor list.
2
Evaluate the impact of an Area ID mismatch on Hello packet processing.
When Area IDs mismatch, incoming Hello packets are dropped on receipt before the neighbor list is processed.
Because Hellos are discarded due to parameter mismatch (such as Area ID, Hello/Dead timers, or subnet mask), bidirectional communication fails to establish, keeping the relationship stuck in INIT.

Key Concept

OSPFv2 Neighbor Adjacencies and Router ID
Estimated Time:1m 0s
Rate this question

Topics

OSPFv2 Neighbor Adjacencies and Router ID

More questions from this topic