Answers to Tshoot Drill: OSPF Hello Timer Changed

 In 200-301 V1 Ch20: OSPF Config, 200-301 V1 Part 6: OSPF, Tshoot Drill

The previous post changed the OSPF Hello interval (timer) on a WAN serial link. What’s that do to the output of various OSPF commands in our familiar router triangle? What happens to show ip ospf neighbor? Show ip ospf interface brief? Show ip route? And on which routers? Today’s post points out the config changes and status changes visible in some of the common show commands.

Earlier posts in this series:

Problem Statement: OSPF Hello Timer Changed

Review of the Problem Statement

Look at the previous post for a full description of the problem statement, including the initial configuration for the various devices. For quick reference, however… the problem begins with a working OSPF configuration on all three routers, with the topology and subnets shown in Figure 1.

Figure 1: Subnets and Topology for this Tshoot Drill

To create this drill, the lab then adds the ip ospf hello-interval 11 command to R1’s S0/0/1 interface (R1’s link connected to R3). What happens?

First Question: Which Show Commands Should I Examine?

One key troubleshooting skill is to think of all the show commands relevant to a particular scenario. You should list those that you know should change because of the changes to the configuration, but also those that are related but do not change. The command output that doesn’t change can sometimes tell you something about a problem as well.

First, ask yourself: should this change affect the state of the interfaces themselves? The answer is no. The interface state of both R1’s S0/0/1 interface, and the matching R3 S0/0/0 interface, stay the same.

Next, ask yourself about the key components of the OSPF configuration: enabled interfaces, neighbors, subnets advertised into the OSPF database, and routes added. Which are affected? Well, it turns out that the mismatched Hello timers between R1 and R3 will prevent them from becoming neighbors.

For this particular exercise, you can check a lot of commands, but the ones that show OSPF neighbor status are the ones to watch for in this case.

R1 Changes: OSPF Neighbors

As stated earlier, the configuration change causes the R1-R3 neighbor relationship to fail, due to the OSPF Hello timer mismatch. (The default setting on R3 is 10; R1’s setting is now 11.)

The show ip ospf neighbor command output in Example 1 shows the change in the most obvious way by no longer listing R3 as a neighbor. , R1’s show ip interface brief command also shows the output. However, I might prefer the show ip ospf interface brief command in this case, because it gives positive information. For R1’s S0/0/1 interface, it shows 0 neighbors, confirming R1 has no neighbors on that interface.  However, by listing S0/0/1 in the output, IOS confirms that OSPF is still enabled on this interface.

Example 1: R2’s S0/0/0 Interface State After the Change

R1#show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface           0   FULL/  -        00:00:33      Serial0/0/0
R1#show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Se0/0/1      1     0           781   P2P   0/0
Se0/0/0      1     0           781   P2P   1/1
Fa0/0        1     0           1     DR    0/0


R3 Changes: OSPF Neighbors

R3 mirrors the same changes in output. R3’s S0/0/0 interface remains enabled with OSPF, but R3’s neighbor relationship with R1 (Router ID no longer exists. Example 2 shows the output for completeness.

Example 2: R3’s OSPF Neighborships

R3# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface           0   FULL/  -        00:00:39     Serial0/0/1
R3# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/0        1     0           1     DR    0/0
Se0/0/0      1     0          64    P2P   1/1
Se0/0/0      1     0           64    P2P   0/0



Answer Part 2: VLSM Design Drill #2
Calculating the Best Summary Route
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Why does this not affect R1 – R2 neighbor relationship?

Wendell Odom

Because the config change on R1 was made on R1’s S0/0/1 interface, which connects to router R3. R3 defaults to 10 second Hello, R1 now uses 11, and the neighbor relationship fails. The R1 interface connected to R2, S0/0/0, remains with the default hello timer of 10, so no issues with R1-R2.

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