5.4 - Troubleshooting network performance
Network+ N10-009 objective 5.4 covers performance problems and the metrics that reveal them. When real-time VoIP has choppy audio although bandwidth is fine, the culprit is jitter - variable delay between packets - best fixed with QoS. When a file transfer is slow and packets arrive out of order with retransmits, the likely cause is packet loss somewhere on the path. A satellite link showing very high round-trip latency despite low loss suffers from propagation delay over the long distance, not congestion. On wireless, a client at the edge with a very low RSSI drops because signal is weak, so the best fix is to improve AP placement and coverage. You should also know bandwidth, throughput and QoS. Expect scenario questions that describe a performance symptom and ask which metric or root cause - jitter, packet loss, latency or low RSSI - explains it.
Choppy VoIP, bandwidth fine = jitter (fix with QoS). Out-of-order + retransmits = packet loss. Very high RTT, low loss (satellite) = propagation delay. Weak edge signal, low RSSI = improve AP placement.
Practice questions
1. Real-time VoIP has choppy audio though bandwidth is fine. Which metric is the culprit?
- Throughput
- MTU size
- Attenuation
- Jitter (correct answer)
Jitter is variation in packet delay; it makes VoIP choppy even with ample bandwidth. Fix with QoS, a jitter buffer, and prioritizing voice traffic.
2. A WLAN edge has weak signal and drops; RSSI is very low. The best fix is to:
- Change the broadcast SSID name
- Disable WPA3 encryption on the AP
- Lower the interface MTU size
- Improve AP placement/coverage (correct answer)
Very low RSSI at the edge means insufficient coverage; add or reposition APs (and adjust power/antennas). SSID name and MTU do not affect signal strength.
3. A file transfer is slow and packets arrive out of order with retransmits. The likely issue is:
- A DNS misconfiguration
- A wrong VLAN
- An MTU that is too small
- Packet loss on the path (correct answer)
Retransmissions and out-of-order delivery point to packet loss (congestion, bad link, errors). Trace the path, check interface error counters and link quality.
4. A satellite link shows very high round-trip latency though loss is low. The cause is:
- A speed and duplex mismatch
- Propagation delay of the distance (correct answer)
- ARP poisoning on the local LAN
- An end-to-end MTU mismatch
Geostationary satellite links have inherent high latency from the long signal distance to orbit and back. It is expected, not a fault; TCP tuning/QoS can help.
5. A trunk carries some VLANs fine but one VLAN can't pass between switches. The likely cause is:
- A speed/duplex mismatch
- A failing SFP transceiver
- VLAN not allowed on the trunk (correct answer)
- A DNS resolution failure
If other VLANs work, the trunk is up; the failing VLAN is likely pruned or not in the allowed list. Add it to the trunk's allowed VLANs on both switches.
6. Users on 5 GHz near several APs report slow Wi-Fi; a scan shows overlapping channels. The cause is:
- Attenuation over distance
- A wrong DNS server
- Co-channel interference (correct answer)
- An MTU mismatch
Overlapping/co-channel APs contend for the same airtime, cutting throughput. Reassign non-overlapping channels and reduce channel width or power. DNS/MTU are unrelated.