Routing Troubleshooting Challenge

Lab Objectives

• Learn to enable and verify OSPF adjacency across multiple routers and ensure networks are advertised.
• Configure and verify a default route to the Internet and ensure internal routers use it.
• Perform basic troubleshooting: identify an OSPF misconfiguration (area/wildcard mistake) and correct it using show commands.

Lab Tasks (Try It Yourself First!)

Complete these tasks WITHOUT looking at the solution below. Use ? and show commands to figure it out.

Task 1: Enable OSPF on all routers

Enable OSPF process 1 on R1, R2, R3, R4. Advertise the directly connected router-to-router networks:

• 10.10.10.0/24 (R1-R2)
• 10.10.20.0/24 (R1-R3)
• 10.10.30.0/24 (R1-R4) Also advertise 10.10.40.0/24 (R2-S1 link) from R2.

Do NOT configure other protocols.

Task 2: Configure default routing to the Internet

• On R1, create a default route to the Internet next-hop 203.0.113.1.
• On R2, R3, R4, configure a static default route pointing to R1 (the 10.10.10.x / 10.10.20.x / 10.10.30.x next-hop as appropriate).

Task 3: Fix an OSPF area / network statement mistake

One router (R3) has its OSPF network statements misconfigured and is not exchanging routes with the rest. Locate and fix the incorrect OSPF configuration so that all routers form adjacencies and all internal routes are visible.

Think About It: Why does a single incorrect network statement or wrong area number prevent routes from being learned, even though physical links are up?

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Lab Solution

(First, the base ASCII topology with exact interface IPs)

                [Internet]
               203.0.113.1
                    |
               R1 (Gateway)
              Gi0/0: 10.10.10.1
              Gi0/1: 10.10.20.1
              Gi0/2: 10.10.30.1
             /     |     \
           R2      R3      R4

Gi0/0:10.10.10.2 Gi0/0:10.10.20.2 Gi0/0:10.10.30.2 Gi0/1:10.10.40.1 | | / \ | | S1 S2 S3 / \ | /
PC1 PC2 PC3 PC4 PC5

Task 1 Solution: Enable OSPF on all routers

What we are doing: Start OSPF process 1 and advertise the local links so routers form adjacencies and exchange route information. OSPF uses Hello packets to discover neighbors and Link-State Advertisements (LSAs) to share routing information — if a network is not in the OSPF configuration it will not be advertised.

! On R1
router ospf 1
 network 10.10.10.0 0.0.0.255 area 0
 network 10.10.20.0 0.0.0.255 area 0
 network 10.10.30.0 0.0.0.255 area 0

What just happened:

• router ospf 1 — creates OSPF process 1.
• network X Y area 0 — tells the router to run OSPF on any interface in that subnet and advertise it into area 0. The wildcard mask 0.0.0.255 matches /24 networks.

Verify:

! Check OSPF neighbors and OSPF-learned routes on R1
show ip ospf neighbor

Expected output (example):

R1#show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.10.10.2       1   FULL/DR         00:00:33    10.10.10.2      GigabitEthernet0/0
10.10.20.2       1   FULL/DR         00:00:33    10.10.20.2      GigabitEthernet0/1
10.10.30.2       1   FULL/DR         00:00:33    10.10.30.2      GigabitEthernet0/2

Also verify routes learned via OSPF:

show ip route ospf

Expected snippet:

O     10.10.20.0/24 [110/20] via 10.10.20.2, GigabitEthernet0/1
O     10.10.30.0/24 [110/20] via 10.10.30.2, GigabitEthernet0/2
O     10.10.10.0/24 is directly connected, GigabitEthernet0/0

Repeat the OSPF configuration on R2, R3, R4 advertising their local links. Example for R2:

! On R2
router ospf 1
 network 10.10.10.0 0.0.0.255 area 0
 network 10.10.40.0 0.0.0.255 area 0

Verify on R2:

show ip route ospf

Expected snippet:

O     10.10.20.0/24 [110/20] via 10.10.10.1, GigabitEthernet0/0
O     10.10.30.0/24 [110/20] via 10.10.10.1, GigabitEthernet0/0
C     10.10.10.0/24 is directly connected, GigabitEthernet0/0
C     10.10.40.0/24 is directly connected, GigabitEthernet0/1

Task 2 Solution: Configure default routing to the Internet

What we are doing: Add a default route on R1 pointing to the Internet next-hop 203.0.113.1. Add static default routes on the internal routers so they forward unknown traffic to R1.

! On R1
ip route 0.0.0.0 0.0.0.0 203.0.113.1

What just happened:

• ip route 0.0.0.0 0.0.0.0 203.0.113.1 — creates a gateway of last resort; any destination not in the routing table will be sent to the Internet next-hop.

Verify on R1:

show ip route

Expected snippet:

Gateway of last resort is 203.0.113.1 to network 0.0.0.0

S*    0.0.0.0/0 [1/0] via 203.0.113.1
C     10.10.10.0/24 is directly connected, GigabitEthernet0/0
...

On R2, point default to R1:

! On R2
ip route 0.0.0.0 0.0.0.0 10.10.10.1

Why this matters: Internal routers need a default route so they can reach external destinations. Without it, packets for unknown networks will be dropped even if OSPF works.

Verify on R2:

show ip route

Expected snippet:

Gateway of last resort is 10.10.10.1 to network 0.0.0.0

S*    0.0.0.0/0 [1/0] via 10.10.10.1
O     10.10.20.0/24 [110/20] via 10.10.10.1, GigabitEthernet0/0
...

Repeat static default on R3 and R4 pointing to their R1 hop (10.10.20.1 or 10.10.30.1 as appropriate).

Task 3 Solution: Fix OSPF area / network statement mistake

What we are doing: Identify R3 misconfiguration and correct the network statement so its link is in area 0 and it advertises connected networks.

Common misconfiguration: R3 configured OSPF with the wrong network wildcard or wrong area (e.g., area 1) so it never forms adjacencies.

! Inspect R3 OSPF config then fix
show running-config | section router ospf

! Correct commands on R3
configure terminal
 router ospf 1
  no network 10.10.20.0 0.0.0.255 area 1
  network 10.10.20.0 0.0.0.255 area 0
end

What just happened:

• show running-config | section router ospf — lets you view OSPF stanza to find mistakes.
• Removing the incorrect network in area 1 and adding it to area 0 ensures R3 participates in the same backbone area as the other routers. OSPF neighbors form only if area and Hello parameters match.

Verify neighbor and route learning on R3:

show ip ospf neighbor
show ip route ospf

Expected:

R3#show ip ospf neighbor
Neighbor ID     Pri   State    Dead Time   Address         Interface
10.10.20.1       1    FULL/DR  00:00:32    10.10.20.1      GigabitEthernet0/0

R3#show ip route ospf
O     10.10.10.0/24 [110/20] via 10.10.20.1, GigabitEthernet0/0
O     10.10.30.0/24 [110/20] via 10.10.20.1, GigabitEthernet0/0

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Troubleshooting Scenario

Scenario: R3 is not exchanging routes

Symptom: Ping from R2 to R4 succeeds, but ping from R3 to Internet fails; show ip ospf neighbor on R3 shows no neighbors.

Your task: Find and fix the issue.

Hint: Check the OSPF network statements and area numbers on R3.

Solution: On R3, use show running-config | section router ospf and show ip ospf neighbor. If R3's network statement used area 1 (or a wrong wildcard) then remove the incorrect statement and add the correct network 10.10.20.0 0.0.0.255 area 0. After correction, ensure show ip ospf neighbor shows FULL state and show ip route shows OSPF-learned networks. This matters because OSPF forms adjacencies only when both sides agree on Hello/Dead timers, area, and network type.

Verification Checklist

• All routers have OSPF process 1 configured and show FULL neighbors.
• R1 has ip route 0.0.0.0 0.0.0.0 203.0.113.1.
• R2/R3/R4 have static default routes pointing to R1.
• show ip route ospf on each router displays the other 10.10.x.0 networks.

Common Mistakes

Symptom	Cause	Fix
No OSPF neighbors	Wrong area number or incorrect network/wildcard	Correct network ... area 0 to match backbone area and ensure wildcard mask matches /24
Router has no default route	Missing ip route 0.0.0.0 0.0.0.0 on R1 or internal routers	Add ip route 0.0.0.0 0.0.0.0 203.0.113.1 on R1 and static default on internals
Routes present but cannot reach Internet	Default route installed but next-hop unreachable	Ping next-hop (203.0.113.1) and verify interface IPs; fix connectivity to next-hop

Challenge Task

Add a new site R5 connected to R2 on subnet 10.10.50.0/24. Advertise that network into OSPF so that all routers learn 10.10.50.0/24. Do not change existing OSPF area design. Your goal: configure minimal commands to make 10.10.50.0/24 visible from R1 and routable to the Internet. (You must determine the correct network statement and where to add it.)