Question 74 - JN0-664 discussion

In OSPF, routers form adjacencies by exchanging Hello packets. These packets contain several parameters that must match between OSPF neighbors to establish a successful adjacency. Key among these parameters are the Hello Interval and the Dead Interval.

Let's analyze the exhibit and the question to determine the correct course of action to ensure that R4 peers with both R1 and R2.

1. **OSPF Hello and Dead Intervals**:

- **Hello Interval**: The time in seconds between Hello packets sent out by a router. Default is usually 10 seconds for broadcast and point-to-point networks.

- **Dead Interval**: The time in seconds that a router will wait to receive a Hello packet from a neighbor before declaring the neighbor down. Default is usually four times the Hello Interval (40 seconds).

2. **Analysis of the Exhibit**:

- **R1's Interface**: Shows a Dead Interval of 40 seconds.

- **R2's Interface**: Shows a Dead Interval of 40 seconds.

- **R4's Interface**: Shows a Dead Interval of 20 seconds.

From the exhibit, we can see that R4 has a Dead Interval of 20 seconds, while R1 and R2 have Dead Intervals of 40 seconds. This discrepancy prevents R4 from establishing an OSPF peering with R1.

3. **Option Analysis**:

- **A. Adjust the Priority on R1 to be lower than the Priority on R4**:

- Incorrect. The OSPF Priority is used for DR/BDR election on multi-access networks and does not impact peering issues.

- **B. Change the MTU size on R1 and R2 to be 22 bytes higher than R4's MTU size**:

- Incorrect. While MTU mismatches can prevent OSPF adjacencies, the exhibit does not indicate an MTU mismatch issue.

- **C. Adjust the Dead Interval on R4 to match the Dead Interval on R1 and R2**:

- Correct. Matching the Dead Interval on R4 to 40 seconds will ensure that all routers have consistent Hello and Dead Intervals, allowing OSPF adjacencies to form.

- **D. Adjust the Hello Interval on R1 and R2 to match the Hello Interval on R4**:

- While this would be a valid approach if the Hello Intervals were mismatched, the exhibit shows that the Hello Intervals are consistent (10 seconds) across all routers. Therefore, this adjustment is not necessary.

**Conclusion**:

To correct the OSPF peering issue, the Dead Interval on R4 should be adjusted to match the Dead Intervals on R1 and R2. The correct answer is:

**C. Adjust the Dead Interval on R4 to match the Dead Interval on R1 and R2.**

**Reference**:

- Juniper Networks Documentation on OSPF: [OSPF Overview](https://www.juniper.net/documentation/en_US/junos/topics/concept/ospf-routing-overview.html)

- OSPF Configuration Guide: [Configuring OSPF](https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/ospf-configuring.html)