NE NE Network Troubleshooting 2 — Questions and Answers
Question 1: What does the command 'show interface' reveal that helps diagnose an interface problem?
- Routing table entries associated with the interface
- Line status, protocol status, error counters, and traffic statistics (Correct answer)
- IP address assignments and subnet mask
- VLAN membership and trunk status
Correct answer: Line status, protocol status, error counters, and traffic statistics
'Show interface' reports line protocol state, input/output errors, drops, CRC errors, and collisions — essential for diagnosing physical and data-link issues.
Question 2: A network engineer sees 'interface is up, line protocol is down' on a serial interface. What does this indicate?
- Physical connectivity exists but the data link layer is not functioning (e.g., encapsulation mismatch) (Correct answer)
- Both physical and logical connectivity are confirmed
- The interface has been administratively shut down
- The IP address is incorrect
Correct answer: Physical connectivity exists but the data link layer is not functioning (e.g., encapsulation mismatch)
Up/down means the physical signal is present (cable connected) but keepalives or LCP negotiation are failing — typically an encapsulation mismatch or clocking issue.
Question 3: Which command shows the current ARP cache on a Cisco router?
- show ip route
- show arp (Correct answer)
- show mac address-table
- show ip interface brief
Correct answer: show arp
'Show arp' displays the router's ARP table mapping IP addresses to MAC addresses for connected interfaces and resolved next-hops.
Question 4: What is the most effective first step when troubleshooting a reported network outage?
- Immediately reboot the affected routers and switches
- Define and document the problem by gathering information from affected users and systems (Correct answer)
- Replace the suspected faulty cable
- Clear all routing tables and re-establish adjacencies
Correct answer: Define and document the problem by gathering information from affected users and systems
Effective troubleshooting begins with defining the problem — understanding scope, symptoms, and recent changes before making any modifications.
Question 5: A network engineer suspects an MTU mismatch is causing packet fragmentation issues. Which command helps verify the MTU on a Cisco interface?
- show ip interface
- show interface [name] (Correct answer)
- show running-config interface
- show ip route
Correct answer: show interface [name]
'Show interface' displays the MTU configured on an interface; mismatched MTUs on adjacent routers cause fragmentation or dropped oversized packets.
Question 6: What does a 'request timed out' response to a ping indicate?
- The destination host is reachable but is rejecting ICMP
- The destination is unreachable or ICMP responses are being blocked (Correct answer)
- The source interface is down
- There is a DNS resolution failure
Correct answer: The destination is unreachable or ICMP responses are being blocked
A timeout means no ICMP Echo Reply was received within the timeout period, indicating the destination is unreachable, too slow, or has ICMP blocked by a firewall.
NE NE Network Troubleshooting 2
What does the command 'show interface' reveal that helps diagnose an interface problem?