Default Route Missing — Remote Users Losing Internet
Presenting Symptom
Remote users report they cannot access the internet, but internal network resources are reachable.
Network Context
A small branch office with a Cisco 4331 router (IOS XE 16.9) connects to the ISP via a serial link (S0/1/0) and to the internal LAN via GigabitEthernet0/0. The router has a default route pointing to the ISP, but it is missing from the routing table. The branch has about 50 users.
Diagnostic Steps
Check the routing table for a default route
show ip routeCodes: L - local, 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 2
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, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override
Gateway of last resort is not set
C 192.168.1.0/24 is directly connected, GigabitEthernet0/0
L 192.168.1.1/32 is directly connected, GigabitEthernet0/0
C 10.0.0.0/30 is directly connected, Serial0/1/0
L 10.0.0.1/32 is directly connected, Serial0/1/0The 'Gateway of last resort is not set' line indicates there is no default route. Normally, you would see 'S* 0.0.0.0/0 [1/0] via <next-hop>' if a static default route is configured.
Verify if a static default route is configured
show running-config | include ip route 0.0.0.0 0.0.0.0No output (empty)
No output means there is no static default route in the running configuration. If present, you would see 'ip route 0.0.0.0 0.0.0.0 <next-hop>'.
Check the configuration of the interface connecting to the ISP
show running-config interface Serial0/1/0interface Serial0/1/0 ip address 10.0.0.1 255.255.255.252 encapsulation ppp no shutdown
The interface is configured with an IP address and is up. This confirms the physical layer is working, but the missing default route is the issue.
Confirm that the ISP next-hop is reachable
ping 10.0.0.2Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
The ping succeeds, so the ISP next-hop is reachable. The problem is purely the missing default route.
Root Cause
The static default route (ip route 0.0.0.0 0.0.0.0 10.0.0.2) was either never configured or was accidentally removed from the router's running configuration.
Resolution
Verification
show ip route | include Gateway Expected output: Gateway of last resort is 10.0.0.2 to network 0.0.0.0 Also verify: show ip route 0.0.0.0 Expected output: Routing entry for 0.0.0.0/0, supernet Known via "static", distance 1, metric 0 (connected) Routing Descriptor Blocks: * 10.0.0.2 Route metric is 0, traffic share count is 1 Finally, test internet access from a remote user's PC: ping 8.8.8.8 should succeed.
Prevention
1. Always verify the routing table after configuration changes. 2. Use a configuration backup and change management process to prevent accidental deletion. 3. Consider using a routing protocol (e.g., BGP) with the ISP to dynamically learn the default route.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where you must identify why a default route is missing. Expect multiple-choice questions asking 'What is the cause of no internet access?' or drag-and-drop to place the correct static route command. The key fact is that a default route (0.0.0.0/0) is required for internet access, and 'show ip route' displays the gateway of last resort.
Exam Tips
Memorize the 'show ip route' output and look for 'Gateway of last resort is not set'.
Know that 'ip route 0.0.0.0 0.0.0.0 <next-hop>' is the correct syntax for a static default route.
Remember that a missing default route causes loss of internet access but internal connectivity remains.
Commands Used in This Scenario
ping [ip]
The ping command sends ICMP echo requests to a destination IP address to test network connectivity and measure round-trip time.
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.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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