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Troubleshooting MethodologyCCNA 200-301

Network Broke After Configuration Change — Rollback Procedure

Presenting Symptom

After applying a configuration change, network connectivity to remote sites is lost and users report inability to reach resources across the WAN.

Network Context

The network is a small branch office with a single Cisco 4321 ISR router connected to an enterprise campus via a serial link (HDLC encapsulation). The router runs IOS XE 16.9 and uses static default routes pointing to the campus router. The branch has a single VLAN 10 with a /24 subnet. The change involved adding a new static route for a test network, but after reload, connectivity broke.

Diagnostic Steps

1

Check interface status and IP connectivity

show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0/0      192.168.1.1     YES NVRAM  up                    up
Serial0/1/0               10.1.1.1        YES NVRAM  up                    up

Verify that the WAN interface (Serial0/1/0) is up/up. If down/down, check cable or carrier. If up/down, check clocking or encapsulation mismatch.

2

Examine the routing table for the default route

show ip route
S*   0.0.0.0/0 [1/0] via 10.1.1.2
     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       10.1.1.0/30 is directly connected, Serial0/1/0
L       10.1.1.1/32 is directly connected, Serial0/1/0
     192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.1.0/24 is directly connected, GigabitEthernet0/0/0
L       192.168.1.1/32 is directly connected, GigabitEthernet0/0/0

Look for the default route (0.0.0.0/0). If missing, the static route was removed or not saved. If present but pointing to wrong next-hop, connectivity fails.

3

Check the running configuration for static routes

show running-config | include ip route
ip route 0.0.0.0 0.0.0.0 10.1.1.2

Confirm the static route is present. If the output is empty, the route was deleted. If the next-hop is incorrect, it may have been overwritten.

4

Verify the startup configuration

show startup-config | include ip route
ip route 0.0.0.0 0.0.0.0 10.1.1.2

Compare running and startup configs. If the route is missing from startup but present in running, the change was not saved (copy run start). If missing from both, the route was removed.

Root Cause

The configuration change added a new static route but inadvertently removed the existing default static route. The new route was saved to startup, but the default route was not re-added. After reload, the router lost the default route, breaking all off-subnet connectivity.

Resolution

Re-add the default static route and save the configuration. 1. Enter global configuration mode: configure terminal 2. Add the default static route: ip route 0.0.0.0 0.0.0.0 10.1.1.2 3. Save the running configuration to startup: copy running-config startup-config Explanation: The ip route command creates a static route to 0.0.0.0/0 (all destinations) via next-hop 10.1.1.2. The copy run start command ensures the change persists after reload.

Verification

1. Verify the routing table: show ip route Expected output includes: S* 0.0.0.0/0 [1/0] via 10.1.1.2 2. Test connectivity: ping 8.8.8.8 Expected: Success rate is 100 percent (5/5) 3. Confirm startup config: show startup-config | include ip route Expected: ip route 0.0.0.0 0.0.0.0 10.1.1.2

Prevention

["Always use the 'copy running-config startup-config' command after any configuration change to ensure changes survive reload.","Before making changes, back up the running configuration using 'copy running-config tftp:' or 'show running-config' to a text file.","Use the 'reload in 10' command to schedule a reload with a 10-minute delay; if connectivity breaks, you can cancel with 'reload cancel' before the reload occurs."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests the candidate's ability to troubleshoot connectivity loss after configuration changes. Questions may present a similar scenario in a multiple-choice format asking for the most likely cause (e.g., missing default route) or the correct fix. The exam emphasizes the importance of saving configurations and understanding the difference between running and startup configs.

Exam Tips

1.

Memorize the 'copy running-config startup-config' command; it is the most common cause of post-reload issues.

2.

Understand that 'show ip route' is the primary command to verify routing; a missing default route is a classic symptom.

3.

Remember that static routes are removed if you enter 'no ip route' or if the configuration is overwritten without the original route.

Commands Used in This Scenario

show ip route

Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.

show running-config

Displays the current active configuration in DRAM, showing all non-default settings.

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