A duplex mismatch leaves the link up and pings working while throughput dies. This post shows the exact syslog message CDP prints when it spots one and how to read every error counter in show interfaces.
Learn, forward, flood: the three rules a switch applies to every frame. Watch MAC addresses enter the CAM table, age, and trigger flooding, with real Cisco IOS XE output from two Linux hosts and a router in VLAN 10.
Subnetting has exactly one right answer and the exam gives you under a minute to find it. Learn the block-size method, walk a VLSM plan the way you would on a real network, and verify it all with real Cisco IOS XE output.
EIGRP has three administrative distances, not one: 90 internal, 170 external, 5 for summaries. Each value encodes a deliberate trust decision. This post explains why they differ and how to read and change them.
We broke a healthy lab path three ways on real Cisco routers. Learn the three fingerprints of slow throughput and the iPerf3 tests that expose each.
Foreground vs daemon mode, the one-test-at-a-time busy error, multi-port servers, systemd units, and the firewall rules an iPerf3 test server actually needs.
Two tools, not two versions: they do not interoperate, flags collide, and each wins different jobs. Real output from iperf 2.2.1 and 3.18 shows exactly where they differ.
Why fast links benchmark slow: bandwidth-delay product, a real 78% throughput collapse from a 64K window at 104 ms RTT, and how parallel streams buy it back.
UDP mode measures what TCP hides: loss, jitter, and reordering. Real lab captures show a clean test, a 29% loss over-drive, and how to find the loss knee on any path.
Every command here ran across a Cisco IOS XE router in my home lab. A 20 Mbit/s policer cut upload from 138 to 19.2 Mbit/s while the -R download test still showed 110, so a one-direction test would have missed it.
The technical and behavioral questions network engineer interviews actually ask, with strong answers, 2026 salary bands, career paths after CCNA, and resume keywords that pass the ATS.
Find every live host on a subnet with a bash one-liner, and when to reach for nmap -sn instead. Why a down result never means the address is free.