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.
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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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SELECTED READING
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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.
Two equal-cost static defaults per edge were enough for control connections, two BFD sessions, and a 5/5 VRF 10 ping, while show ip bgp summary said BGP not active. Add CE-PE BGP only when the PE expects it, and permit allow-service bgp first.
VRRP only watches the LAN port, so a primary edge that loses every OMP session keeps the gateway unless you track OMP. On 17.18.1a and later, enabling Track OMP writes track omp decrement 10 instead of track omp shutdown.