Answers to Tshoot Drill – WAN Interface Down

The #CCNA scenario begins with a working router triangle, using a single OSPF area. Then you shutdown a serial link. What changes with OSPF show command output? 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: WAN Interface Down Tshoot Drill

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 shuts down R2’s S0/0/0 interface. 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.

For this particular exercise, I would suggest show commands for a couple of topics. First, what commands show the changes in status on the ends of the serial link? Then, what commands show the resulting changes to OSPF. Finally, take a look at the IP routing table. The commands could include:

• Show interfaces
• Show ip interface brief
• Show ip ospf interface brief
• Show ip ospf
• Show ip ospf neighbors
• Show ip ospf database
• Show ip route
• Show ip route ospf

R2 Changes: Interface State

First off, R2’s interface state changes on R2’s shutdown interface, S0/0/0. But what actually changes in the show command output, on both R2, as well as R, which sits on the other end of the serial link?

The most obvious status exists on R2, where the shutdown command is added to S0/0/0. Example 1 shows the state as “administratively down” (meaning shutdown was used) in the show interfaces command; this same state (not shown) exists in the output of the show ip interface brief command output. Example R2 also shows a little common notice of the interface state, out on the far right in the show ip ospf interface brief command, which simply lists S0/0/0 as “DOWN”, making no distinction of the down state being administrative.

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

R2# show interfaces s0/0/0
Serial0/0/0 is administratively down, line protocol is down 
  Hardware is GT96K Serial
  Internet address is 172.16.10.2/23
! Lines omitted for brevity
R2# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Se0/0/0      1     0               172.16.10.2/23     64    DOWN  0/0
Se0/0/1      1     0               172.16.6.2/23      64    P2P   1/1
Fa0/0        1     0               172.16.2.2/23      1     DR    0/0

In comparison, R3 has no awareness of the “administratively down” perspective, because R3’s interface (S0/0/1) is not shutdown. Instead, sees the physical link as down, so both the first and last interface status (per show interfaces) lists a state of “down”. Before R2 shutdown its S0/0/0 interface, this command listed a state of up for both status codes.

Example 2: R3’s Serial Interface on the Other End

R3# show interfaces s0/0/1
Serial0/0/1 is down, line protocol is down 
  Hardware is GT96K Serial
  Internet address is 172.16.10.3/23
! Lines omitted for brevity
R3# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/0        1     0               172.16.4.3/23      1     DR    0/0
Se0/0/1      1     0               172.16.10.3/23     64    DOWN  0/0
Se0/0/0      1     0               172.16.8.3/23      64    P2P   1/1

Also, note that the show ip ospf interface brief command lists the same status of down, which basically matches the second of the two status codes for the interface.

R1 Non-Changes: OSPF Neighbors and Interfaces

R1 should not list any changes in OSPF command output in this particular drill. The serial link that fails sits between R2 and R3. R1’s two neighbor relationships, to R2 and R3, still work. The OSPF-enabled interfaces on R1 (as listed per show ip ospf interfaces) remains the same.

However, R1 does lose one IP route. The R2-R3 serial link itself, subnet 172.16.10.0/23, fails, so none of the three routers will list IPv4 routes for that subnet in the output of show ip route.

Example 3: R1’s show ip ospf neighbor After the Change

R1#show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           0   FULL/  -        00:00:33    172.16.8.3      Serial0/0/1
2.2.2.2           0   FULL/  -        00:00:32    172.16.6.2      Serial0/0/0
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
       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
     172.16.0.0/23 is subnetted, 5 subnets
C       172.16.8.0 is directly connected, Serial0/0/1
O       172.16.4.0 [110/782] via 172.16.8.3, 00:45:51, Serial0/0/1
C       172.16.6.0 is directly connected, Serial0/0/0
C       172.16.0.0 is directly connected, FastEthernet0/0
O       172.16.2.0 [110/782] via 172.16.6.2, 1w6d, Serial0/0/0
R1# show ip route ospf
     172.16.0.0/23 is subnetted, 5 subnets
O       172.16.4.0 [110/782] via 172.16.8.3, 00:46:00, Serial0/0/1
O       172.16.2.0 [110/782] via 172.16.6.2, 1w6d, Serial0/0/0

R2 Changes: OSPF Neighbors and What Else?

R2’s S0/0/0, which connects to R3, fails. R2’s OSPF neighbor relationship with R3 fails. What else changes in show command output?

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. R2’s S0/0/0, which connects to R3, fails. What happens to R2’s OSPF neighbor relationships, as shown with show ip ospf neighbor?
2. What differences (if any) occur in the output of the show ip ospf interface brief command output?
3. How many routes will be listed in R3’s show ip route ospf, as compared to what you saw in Example 2 for R1?