GUIDES MEET PRACTICE
Clear networking guides and 68 CCNA browser labs. Learn the concepts, practice IOS-style commands, and check your work as you go.
One lab free. No account, installation, or hardware needed.
A browser simulation of selected IOS behavior. It does not run an IOS image.
EXPLORE THE LIBRARY
Models, addresses, and the basics that connect everything.
↗ 02Switching & VLANsBuild and troubleshoot Layer 2 networks.
↗ 03RoutingUnderstand forwarding decisions and paths.
↗ 04Security & accessExplore controls, authentication, and firewalls.
↗ 05WirelessFrom WLAN fundamentals to controller operations.
↗ 06Linux & toolsDiagnose and verify from the command line.
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Two hosts on 192.0.2.20. On an access port arpwatch logged nothing while the requester cached the impostor 5 of 6 times. On a SPAN port the same 6 lookups produced 9 flip flop reports. Install, service setup, every report message, and exact errors.
fping -g sweeps a /24 in seconds. Through a 64 kbps ICMP policer, its 10 ms default interval reported 20% to 40% loss on 13 of 21 healthy hosts while the router counted 203 drops. At -i 20: 0% loss, 0 drops. Commands, output and the math.
Through a 200 kbps policer, masscan at --rate 300 found all 64 open ports and at --rate 1000 only 21, because masscan never resends a dropped SYN. nmap -T4 found all 64 in 16 seconds. Commands, output and how to size the rate to the path.
One SYN, four answers: SYN-ACK, RST, ICMP 3/13 from the router, or nothing. I ran hping3 against an IOS XE ACL, found the deny with the router's own unreachables and counters, and hit IOS's 500 ms rate limit: 20 probes, 4 answers.
1% loss dropped 145 segments of a 20 MB download. Wireshark at the client flagged 144 gaps, a capture upstream of the drop counted exactly 145 retransmissions, and the download went from 0.53 s to 26.04 s. Capture, filter and read it step by step.
A least-privilege automation user created 24 hosts through the Checkmk Community 2.5.0p18 REST API; official MCP is not in this edition. Interface 3, the provider A link, went CRITICAL 38 seconds after a shutdown once SNMP helper timeouts were fixed.
With parents set, cutting branch 2 off both providers made the branch edge DOWN and its user UNREACHABLE. A provider A shutdown left every host UP and went HARD CRITICAL on a data-plane check 3 min 51 s later. Real object config and logs.
Cacti 1.2.31 graphed 15 Cisco IOS-XE devices over SNMPv3. A 13-minute provider A shutdown showed as status 2 on one interface graph while the loopback-polled router kept polling. Config, rrdtool checks, and how 5-minute polls blur the edges.
Zabbix 7.0 LTS on Cisco IOS-XE: polling Loopback0 turned a provider A shutdown into an interface-down problem 14 seconds later, instead of a dead-router alert after 2 minutes. Real SNMPv3 config, the ifName fix, and both test runs.
Upgrade Manager by itself, then the Validator, the Controller, and edges in waves. Record show software on every box first: in this 20.18.2.1 and 17.18.02 lab each role had exactly one image, active and default, with no previous version.
VPN 10's LAN prefixes never appear in the VPN 0 table, OMP carries them with label 1004, and a VRF 10 ping crossed the overlay 5/5. How a second service VPN and SGT inline tagging plug into the same design.
MRF keeps edge tunnels inside an access region and sends inter-region traffic through border routers in three hops. A flat 20.18 overlay shows what MRF off looks like: device-role edge-router, region-id-set N/A, and REGION ID None on every OMP path.