Answers to Tshoot Drill

 In 200-301 V1 BBB Not in new books, Tshoot Drill

The last post set up a #CCNA troubleshooting drill. Today, we’ll walk through the analysis. What were your predictions? On which routers (R1, R2, and R3) do you expect to see a change to the output of show ip ospf neighbor? Show ip ospf interface brief? Show ip route? And do those changes point out config changes, or status changes?

Earlier posts in this series:

CCNA Troubleshooting Self-study Exercises

Constructing CCNA Config Drills for Troubleshooting Practice

CCNA Tshoot Config Exercises – Answers

How to Create CCNA Troubleshooting Drills

Recap of the Series

Today’s post makes no sense if you’ve not been following the series. Here’s a quick recap to set the context.

I’ve suggested two types of activities that you can do for self-study to build troubleshooting skills. These drills overcome the issue of self-study in which to make your own exercise for troubleshooting, you spoil the answer for yourself.

The first exercises is to make up short configuration exercises, one that are similar to the config museum exercises already in my both my CCENT and CCNA blogs. These exercises actually help you prepare for Sim questions.

Then, to practice for Simlet questions, you need to exercise your skills with show commands. To do that, take the end result of your config exercise, and remove or change one config command. Then predict the changes in show command output you expect to see as a result. Of course, it’s always better to try these commands on real gear, or a simulator, and so on. The previous post posed one such exercise… and the rest of this post discusses the answers!

One Last Chance to Ponder: Which Commands will Change?

Before I list the answers to the Tshoot drill exercise detailed in the previous post, here’s one last chance to think about the answers. First, consider these three commands, each of which ought to leap to mind when you see a Sim or Simlet question that is clearly about IPv4 routing with OSPF as the routing protocol:

  1. Show ip ospf neighbor
  2. Show ip ospf interface brief
  3. Show ip route

I’ve numbered them arbitrarily, by the way.

Now re-think the config (per this post), and the change to the config (per the previous post). Then think about the three routers in the figure: R1, R2, and R3. Which of these commands’ output changes? In what ways? And which do not?



R1 Changes: IP Routes

Interestingly, R1’s only changed output in this case is command #3, show ip route. First, a few details on why, followed by some comments on why the other two commands do not change.

This Tshoot drill removes R3’s network area 0 config command. As a result, R3 no longer enables OSPF on R3’s F0/0 interface. That means:

  • R3 does not attempt to form OSPF neighbor relationships using F0/0
  • R3 does not advertise the subnet connected to F0/0

That second point results in R3 no longer advertising a route for, meaning that R1 no longer has a route for that subnet in its IP routing table. Example 1 shows R1’s output; note the absence of that route:

Example 1: R1’s show ip route After the Change

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2
 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 ia - IS-IS inter area, * - candidate default, U - per-user static route
 o - ODR, P - periodic downloaded static route
Gateway of last resort is not set is subnetted, 5 subnets
C is directly connected, Serial0/0/1
O [110/845] via, 1d23h, Serial0/0/1
 [110/845] via, 1d23h, Serial0/0/0
C is directly connected, Serial0/0/0
C is directly connected, FastEthernet0/0
O [110/782] via, 1d23h, Serial0/0/0


R1 Non-Changes: OSPF Neighbors and Interfaces

Interestingly, R1 does not lose any OSPF neighbor relationships in this Tshoot drill, so as a result, the output of R1’s show ip ospf neighbor command remains unchanged. The removed network command on R3 does not affect R3’s two serial interface OSPF configuration, so R3 still forms neighbor relationships with both R1 and R2. Example 2 shows the output for reference.

Example 2: R1’s show ip ospf neighbor After the Change

R1#show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface           0   FULL/  -        00:00:33      Serial0/0/1           0   FULL/  -        00:00:32      Serial0/0/0


The show ip ospf interface brief command lists status and counter variables for each OSPF-enabled interface on that local router. By definition, the list of interfaces is a list of interfaces on the local router. So, in this case with R1, with no changes to R1’s configuration, the list of interfaces could not possibly have changed as a result of changed configuration on R3. As it turns out, none of the status values in R1’s output for this command change, either. Example 3 lists the output for reference.

Example 3: R3’s show ip ospf interface brief After the Change

R1#show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Se0/0/1      1     0           781   P2P   1/1
Se0/0/0      1     0           781   P2P   1/1
Fa0/0        1     0           1     DR    0/0


Closing Questions

That’s enough analysis to whet your appetite. To close, consider these questions, and feel free to post your opinions and answers.

  1. Will R2’s show commands change differently than R1’s, or the same?
  2. Will R3’s show commands change differently than R1’s, or the same?
  3. If different, explain your reasoning!


How to Create #CCNA Troubleshooting Drills
#CCNA Tshoot Drill: OSPF WAN Interface Down
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