Verification and Diagnostic Guide: PTL-0014

Lab ID: PTL-0014
Lab Title: Default Gateway Operation and Troubleshooting
Category: network-fundamentals
Simulator Verification: tested_pass

This document details the test procedures, expected outputs, and diagnostic steps required to verify the complete operation of the default gateway and systematically troubleshoot common host and router failure modes.

1. Physical and Logical Topology Verification#

Confirm that all interfaces are active (green link indicators) and correctly assigned:

2. Command-by-Command Verification Matrix#

Test 1: Router Interface Status and Directly Connected Subnets#

Execute on R1:

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol 
GigabitEthernet0/0/0   192.168.10.1    YES manual up                    up 
GigabitEthernet0/0/1   192.168.20.1    YES manual up                    up

Verify the routing table:

R1# show ip route
Gateway of last resort is not set

     192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.10.0/24 is directly connected, GigabitEthernet0/0/0
L       192.168.10.1/32 is directly connected, GigabitEthernet0/0/0
     192.168.20.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.20.0/24 is directly connected, GigabitEthernet0/0/1
L       192.168.20.1/32 is directly connected, GigabitEthernet0/0/1

Test 2: Local Intra-Subnet Communication#

From PC1, ping PC2:

C:\>ping 192.168.10.11
Reply from 192.168.10.11: bytes=32 time<1ms TTL=128

• Success Criteria: 0% packet loss.
• Diagnostic Note: TTL=128 indicates that the packet remained inside the local Layer 2 broadcast domain and was forwarded directly by switch S1 without traversing a router hop.

Test 3: Router Default Gateway Ping#

From PC1, ping the local gateway interface:

C:\>ping 192.168.10.1
Reply from 192.168.10.1: bytes=32 time<1ms TTL=255

• Success Criteria: 0% packet loss.
• Diagnostic Note: TTL=255 confirms the packet was terminated directly by Cisco IOS on router R1.

Test 4: Remote Routed Inter-Subnet Communication#

From PC1, ping PC-Remote:

C:\>ping 192.168.20.10
Reply from 192.168.20.10: bytes=32 time<1ms TTL=127

• Success Criteria: 0% packet loss.
• Diagnostic Note: TTL=127 proves that R1 successfully routed the packet across the network boundary, decrementing the host's initial TTL (128) by 1.

Test 5: Host ARP Resolution Analysis#

From PC1, inspect the resolved ARP table:

C:\>arp -a
  Internet Address      Physical Address      Type
  192.168.10.1          0001.c748.1701        dynamic
  192.168.10.11         00e0.b0c2.31cb        dynamic

• Analysis:
  • 192.168.10.11 resolves to the MAC address of PC2.
  • 192.168.10.1 resolves to the MAC address of R1 Gi0/0/0.
  • Remote host 192.168.20.10 is absent from the ARP cache, proving that end-hosts never resolve MAC addresses across routed boundaries.

3. Systematic Fault Injection and Isolation#

Fault Scenario A: Missing Default Gateway#

1. Set PC1 default gateway to 0.0.0.0.
2. Ping local peer 192.168.10.11: PASS (Local switching works).
3. Ping remote host 192.168.20.10: FAIL (Request timed out / Destination host unreachable).
4. Resolution: Restore gateway to 192.168.10.1.

Fault Scenario B: Incorrect Default Gateway IP#

1. Set PC1 default gateway to unassigned IP 192.168.10.254.
2. Ping remote host 192.168.20.10: FAIL (Request timed out waiting for ARP reply for .254).
3. Resolution: Correct gateway IP to 192.168.10.1.

Fault Scenario C: Gateway Interface Administratively Down#

1. On R1, enter interface GigabitEthernet0/0/0 and execute shutdown.
2. Ping local peer 192.168.10.11 from PC1: PASS (Layer 2 frame switching on S1 is completely unaffected by router state).
3. Ping remote host 192.168.20.10: FAIL.
4. Resolution: On R1, execute interface GigabitEthernet0/0/0 followed by no shutdown.

4. Deliverable File Verification#

Ensure all assets exist within the repository:

• assets/starter.pkt: Unconfigured gateway on PC1.
• assets/completed.pkt: Correct gateway configured on PC1.
• assets/topology.png: Visual diagram of network topology.
• configs/R1.txt: Router baseline configuration.
• configs/S1.txt: Switch 1 management baseline.
• configs/S2.txt: Switch 2 management baseline.