ICND1 Troubleshooting 2 — Questions and Answers
Question 1: A router shows 'Serial0/0 is down, line protocol is down.' What is the FIRST step to troubleshoot this issue?
- Reload the router
- Check the physical cable and CSU/DSU (Correct answer)
- Replace the serial interface module
- Configure the IP address on the interface
Correct answer: Check the physical cable and CSU/DSU
A down/down state indicates a Layer 1 physical problem, so checking the cable and CSU/DSU is the appropriate first step.
Question 2: Which command would you use to verify that CDP neighbors are being detected on a Cisco switch?
- show ip neighbors
- show cdp neighbors (Correct answer)
- show arp neighbors
- show mac-address-table
Correct answer: show cdp neighbors
'show cdp neighbors' displays directly connected Cisco devices discovered via the Cisco Discovery Protocol.
Question 3: A host can ping its default gateway but cannot reach remote networks. What should you check on the router?
- ARP table on the host
- Routing table on the router (Correct answer)
- MAC address table on the switch
- DNS server configuration on the host
Correct answer: Routing table on the router
If the gateway is reachable but remote networks are not, the router's routing table likely lacks the necessary routes.
Question 4: The output of 'show interface FastEthernet0/0' shows 'input errors' incrementing. What does this most likely indicate?
- Duplex mismatch or bad cable causing CRC errors (Correct answer)
- Incorrect IP address configured
- Missing default route
- High CPU utilization on the router
Correct answer: Duplex mismatch or bad cable causing CRC errors
Input errors, especially CRC errors, are commonly caused by duplex mismatches or physical layer problems like damaged cables.
Question 5: You issue 'ping 10.1.1.1' from a router and receive 'U.U.U.' in the output. What do the 'U' characters indicate?
- The destination is unreachable (Correct answer)
- The ping timed out
- The route is unknown
- UDP packets were used instead of ICMP
Correct answer: The destination is unreachable
A 'U' in ping output represents an ICMP Destination Unreachable message received back from a router.
Question 6: A technician runs 'show ip route' and sees a route marked with 'C'. What does the 'C' designation mean?
- The route was learned via OSPF
- The route is a directly connected network (Correct answer)
- The route is a candidate default route
- The route was learned via RIP
Correct answer: The route is a directly connected network
The 'C' code in the routing table indicates a directly connected network on an active interface.
Question 7: Which Cisco IOS command displays the Layer 2 to Layer 3 address mappings learned by a router?
- show mac-address-table
- show ip arp (Correct answer)
- show ip route
- show cdp neighbors detail
Correct answer: show ip arp
'show ip arp' displays the ARP cache, which maps IP addresses to MAC addresses on connected segments.
ICND1 Troubleshooting 2
A router shows 'Serial0/0 is down, line protocol is down.' What is the FIRST step to troubleshoot this issue?