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Lesson 3 of 5

Layer 3 Troubleshooting

Lab Objectives

  • Configure Layer 3 IP addressing on the routers in the base topology and bring interfaces up.
  • Enable EIGRP (AS 100) to provide routing between the routers so remote subnets are reachable.
  • Diagnose and fix a common Layer 3 problem: a mismatched subnet mask that prevents routing and reachability.

Tip: In this lesson you'll practice the essentials of Layer 3 troubleshooting — IP addressing, subnet masks, interface state, and routing protocol adjacency. Think of IP addressing like postal addresses — if the numbers or masks are wrong, packets are delivered to the wrong "house" or never leave the local network.

ASCII Topology (use this EXACT topology and 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           S3  S3
          /  \    |        /  \         /  \  /  \
        PC1 PC2  PC3     PC4  PC5     PC4 PC5 PC6

IP Scheme (networks used in the lab)

  • 10.10.10.0/24 — R1 ↔ R2 link
  • 10.10.20.0/24 — R1 ↔ R3 link
  • 10.10.30.0/24 — R1 ↔ R4 link
  • 10.10.40.0/24 — R2 ↔ S1 link
  • 192.168.1.0/24 — VLAN 10 (Sales)
  • 192.168.2.0/24 — VLAN 20 (Engineering)
  • 192.168.3.0/24 — VLAN 30 (Management)
  • 203.0.113.0/24 — Public/Internet simulation (R1 loopback will simulate Internet)

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: Configure router IP interfaces

Assign the IP addresses for R1–R4 interfaces using the exact IPs from the topology. On R1 also create Loopback0 with IP 203.0.113.1/24 to simulate the Internet.

Task 2: Enable EIGRP for routing

Enable EIGRP with autonomous system 100 on R1–R4 and advertise the directly-connected networks so each router learns all remote subnets.

Task 3: Verify connectivity

From R2, ping the simulated Internet address 203.0.113.1 and verify routes on each router's routing table.

Think About It: If R2 can ping 10.10.10.1 but cannot ping 203.0.113.1, where might the problem be located — on the local router, on the routing table, or across the network? Why?

Lab Solution

Task 1 Solution: Configure router IP interfaces

What we are doing: Assign the exact Layer 3 addresses to each router interface and bring interfaces up. This establishes the Layer 3 topology so routing can operate. In production, correct addressing ensures devices are in the expected subnets — a mismatch prevents routing and host reachability.

!--- R1: configure physical interfaces and a loopback to simulate Internet
R1(config)#interface GigabitEthernet0/0
R1(config-if)#ip address 10.10.10.1 255.255.255.0
R1(config-if)#duplex full
R1(config-if)#no shutdown

R1(config)#interface GigabitEthernet0/1
R1(config-if)#ip address 10.10.20.1 255.255.255.0
R1(config-if)#duplex full
R1(config-if)#no shutdown

R1(config)#interface GigabitEthernet0/2
R1(config-if)#ip address 10.10.30.1 255.255.255.0
R1(config-if)#duplex full
R1(config-if)#no shutdown

R1(config)#interface Loopback0
R1(config-if)#ip address 203.0.113.1 255.255.255.0

What the commands do and why they matter:

  • interface GigabitEthernetX/Y — selects the physical interface to configure.
  • ip address <addr> <mask> — assigns the Layer 3 address; critical because routers use these addresses for neighbor adjacency and routing.
  • duplex full — ensures proper link duplex (mirrors the reference style); mismatched duplex can cause packet loss.
  • no shutdown — administratively enables the interface; an interface that is shutdown will never forward packets.
  • interface Loopback0 + ip address — a loopback simulates a stable host (Internet) for testing; loopbacks are always up and often used in production for stable addresses.

Verify:

R1#show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.10.1      YES manual up                    up
GigabitEthernet0/1     10.10.20.1      YES manual up                    up
GigabitEthernet0/2     10.10.30.1      YES manual up                    up
Loopback0              203.0.113.1     YES manual up                    up

Repeat the interface configuration on R2, R3, and R4 exactly as the topology prescribes:

!--- R2
R2(config)#interface GigabitEthernet0/0
R2(config-if)#ip address 10.10.10.2 255.255.255.0
R2(config-if)#duplex full
R2(config-if)#no shutdown

R2(config)#interface GigabitEthernet0/1
R2(config-if)#ip address 10.10.40.1 255.255.255.0
R2(config-if)#duplex full
R2(config-if)#no shutdown

!--- R3
R3(config)#interface GigabitEthernet0/0
R3(config-if)#ip address 10.10.20.2 255.255.255.0
R3(config-if)#duplex full
R3(config-if)#no shutdown

!--- R4
R4(config)#interface GigabitEthernet0/0
R4(config-if)#ip address 10.10.30.2 255.255.255.0
R4(config-if)#duplex full
R4(config-if)#no shutdown

Verify each router (show ip interface brief) — you should see the interfaces up/up with the assigned addresses.

Task 2 Solution: Enable EIGRP (AS 100) and advertise networks

What we are doing: Start EIGRP on each router and advertise the networks they are directly attached to. EIGRP will exchange routing information so every router can reach all remote subnets. In production, a dynamic routing protocol like EIGRP (or OSPF) eliminates the need for many static routes and adapts to topology changes.

!--- R1
R1(config)#router eigrp 100
R1(config-router)#network 10.10.10.0
R1(config-router)#network 10.10.20.0
R1(config-router)#network 10.10.30.0
R1(config-router)#network 203.0.113.0

!--- R2
R2(config)#router eigrp 100
R2(config-router)#network 10.10.10.0
R2(config-router)#network 10.10.40.0

!--- R3
R3(config)#router eigrp 100
R3(config-router)#network 10.10.20.0

!--- R4
R4(config)#router eigrp 100
R4(config-router)#network 10.10.30.0

What the commands do and why they matter:

  • router eigrp 100 — enables EIGRP with autonomous system number 100. The AS number must match on routers that should form adjacencies.
  • network <network> — tells EIGRP to advertise that directly-connected network and to form adjacencies on interfaces in that network. If a network is omitted, the router will not advertise or form neighbors on that segment.

Verify EIGRP adjacency:

R1#show ip eigrp neighbors
IP-Address      Interface       Hold Uptime   SRTT   RTO  Q  Seq
10.10.10.2      Gi0/0           12   00:00:10  20     100  0  3
10.10.20.2      Gi0/1           11   00:00:08  16     100  0  5
10.10.30.2      Gi0/2           12   00:00:09  24     100  0  2

Verify routes learned:

R2#show ip route
Codes: C - connected, D - EIGRP
C    10.10.10.0/24 is directly connected, GigabitEthernet0/0
C    10.10.40.0/24 is directly connected, GigabitEthernet0/1
D    10.10.20.0/24 [90/307200] via 10.10.10.1, 00:00:12, GigabitEthernet0/0
D    10.10.30.0/24 [90/307200] via 10.10.10.1, 00:00:12, GigabitEthernet0/0
D    203.0.113.0/24 [90/307200] via 10.10.10.1, 00:00:12, GigabitEthernet0/0

Task 3 Solution: Verify connectivity (ping)

What we are doing: Test reachability from R2 to the simulated Internet loopback on R1. Successful pings prove that IP addressing and routing are correct.

R2#ping 203.0.113.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 203.0.113.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/12 ms

What this shows and why it matters:

  • Each ! indicates a successful ICMP echo reply. If pings fail, use show ip route, show ip interface brief, and show ip eigrp neighbors to determine where the break occurred (interface down, no route, or no adjacency).

Troubleshooting Scenario

Scenario: R3 cannot reach the Internet

Symptom: From R3, ping to 203.0.113.1 fails while pings to its immediate neighbor R1 (10.10.20.1) succeed.

Your task: Find and fix the issue.

Hint: Check the IP configuration and subnet mask on R3's interface.

Solution:

  • Likely cause: R3's Gi0/0 was configured with the wrong subnet mask (for example 255.255.0.0). That will still allow local ping to the .1 address in some cases but will prevent correct routing and EIGRP adjacency because the router believes it is in a different network.

Fix commands (example incorrect mask -> corrected):

!--- Examine interface
R3#show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.20.2      YES manual up                    up

!--- Correct the subnet mask if it was wrong:
R3(config)#interface GigabitEthernet0/0
R3(config-if)#ip address 10.10.20.2 255.255.255.0
R3(config-if)#no shutdown

Verify the fix:

R3#show ip route
D    203.0.113.0/24 [90/307200] via 10.10.20.1, 00:00:10, GigabitEthernet0/0

R3#ping 203.0.113.1
!!!!!
Success rate is 100 percent (5/5)

Important note: Mismatched masks are one of the most common causes of routing problems. Even if two routers share addresses that look similar, a wrong mask means they are logically in different networks.

Verification Checklist

  • Interfaces on R1–R4 show the correct IP addresses and Status/Protocol = up/up (show ip interface brief).
  • EIGRP neighbors are established on each adjacent link (show ip eigrp neighbors).
  • All routers have routes to the other LANs and to 203.0.113.0/24 in their routing table (show ip route).
  • Ping from R2 (or R3/R4) to 203.0.113.1 succeeds.

Common Mistakes

SymptomCauseFix
Router cannot form EIGRP neighbor with adjacent routerMismatched network statements or autonomous system numberEnsure router eigrp 100 is used on both routers and the network statement covers the interface IP
Ping to remote loopback fails although local neighbor ping worksWrong subnet mask on local interfaceCorrect the mask with ip address <addr> <mask> on the interface and no shutdown
Route to remote network missing from routing tableNetwork not advertised in EIGRP (missing network command)Add the missing network <network> under router eigrp 100 and verify neighbors

Challenge Task

Without step-by-step guidance: Add a new Router R5 connected to R2 on a new subnet 10.10.50.0/24 (R2 Gi0/2 = 10.10.50.1, R5 Gi0/0 = 10.10.50.2). Configure R5, enable EIGRP AS 100, and verify that R5 can ping 203.0.113.1. Document the commands you used and verification outputs.

Real-world context: In production, these same troubleshooting steps — checking interface state, verifying addressing and masks, and validating routing protocol adjacencies — are foundational. A systematic approach (ping, traceroute, show interface, show ip route, show ip protocols) quickly isolates whether the issue is physical, addressing, or routing. Think of it like detective work: confirm each assumption (is the interface up? is the IP correct?) before moving on.

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