3.7 Troubleshooting IP Connectivity
Key Takeaways
- Use a structured approach: verify Layer 1 (physical), Layer 2 (data link), then Layer 3 (network).
- Ping tests Layer 3 connectivity; traceroute identifies where packets are being dropped.
- Common routing issues: missing routes, incorrect subnet masks, wrong next-hop, passive interface blocking OSPF.
- ARP resolution failures cause connectivity issues even when routes are correct.
- The 'show ip interface brief' command quickly identifies interface status and IP addressing issues.
Troubleshooting IP Connectivity
Troubleshooting is a critical skill for the CCNA exam. You must be able to systematically identify and resolve network connectivity issues.
Structured Troubleshooting Approach
Bottom-Up (Layer 1 → Layer 7)
Layer 1 (Physical):
- Is the cable connected? Check link lights.
- Is the interface up/up? (
show ip interface brief) - Speed/duplex mismatch? (
show interfaces) - CRC errors, input/output errors? (
show interfaces)
Layer 2 (Data Link):
- Is the MAC address in the switch's MAC table? (
show mac address-table) - Is the port in the correct VLAN? (
show vlan brief) - Is the trunk working? (
show interfaces trunk) - STP blocking the port? (
show spanning-tree) - Is ARP resolving? (
show ip arp)
Layer 3 (Network):
- Is the IP address correct? (
show ip interface brief) - Is the subnet mask correct?
- Is there a route to the destination? (
show ip route) - Is the default gateway reachable? (
ping) - ACL blocking traffic? (
show access-lists) - NAT working? (
show ip nat translations)
Essential Troubleshooting Commands
| Command | What It Shows |
|---|---|
show ip interface brief | Interface status and IP addresses (quick summary) |
show interfaces | Detailed interface stats, errors, speed/duplex |
show ip route | Routing table |
show ip arp | ARP cache (IP to MAC mappings) |
show mac address-table | Switch MAC address table |
show vlan brief | VLAN port assignments |
show cdp neighbors | Directly connected Cisco devices |
show ip ospf neighbor | OSPF neighbor adjacencies |
show access-lists | ACL rules and match counts |
ping <ip> | Test Layer 3 reachability |
traceroute <ip> | Show hop-by-hop path to destination |
Interface Status Combinations
| Status | Meaning | Likely Cause |
|---|---|---|
| up/up | Interface is operational | Normal |
| up/down | Layer 1 up, Layer 2 down | Encapsulation mismatch, keepalive failure |
| down/down | Interface is down | No cable, bad cable, remote side shut down |
| administratively down/down | Interface has been shut down | shutdown command was entered |
Common Connectivity Problems and Solutions
| Symptom | Possible Cause | Verification Command | Fix |
|---|---|---|---|
| Can't ping default gateway | Wrong IP/mask, interface down | show ip interface brief | Fix IP, no shutdown |
| Can ping gateway, can't ping remote host | Missing route | show ip route | Add static route or fix OSPF |
| Intermittent connectivity | Duplex mismatch | show interfaces (check errors) | Set both sides to auto or full |
| One VLAN works, another doesn't | Trunk not allowing VLAN | show interfaces trunk | Add VLAN to allowed list |
| OSPF neighbor not forming | Timer mismatch, passive interface, area mismatch | show ip ospf neighbor, show ip ospf interface | Match timers, remove passive, fix area |
Ping Extended Options
Router# ping
Protocol [ip]:
Target IP address: 10.1.1.1
Repeat count [5]: 100
Datagram size [100]: 1500
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface: 192.168.10.1 ! Specify source IP
Specifying a source address is critical when testing routing—the return traffic must have a route back to your source address.
On the Exam: Always think about BOTH directions of traffic. If Router A can reach Router B, but Router B doesn't have a return route to the source, the ping will fail. This is one of the most common troubleshooting scenarios.
A router interface shows status "up/down". What is the most likely cause?
You can ping a host from the router but cannot ping a remote server. The routing table shows no route to the server network. What should you do?
Which command shows a quick summary of all interfaces, their IP addresses, and their up/down status?