Duplex Mismatch and Interface Errors: Find It in 10 Seconds
"The network is slow" tickets that turn out to be a duplex mismatch are a rite of passage.
Troubleshooting
Broken networks, reproduced on purpose. Articles tagged Troubleshooting walk a fault from symptom to root cause using show commands, captures and a repeatable method.
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Ping
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Linux Ping Options Every Network Engineer Should Know
The iputils ping flags engineers actually use: -c/-i/-q for jitter, -s/-M for MTU, -D for timestamps, and the -I flag that fixes confusing IPv6 failures.
Ping
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Testing MTU With Ping and the DF Bit
Find the path MTU with nothing but ping, from Linux and IOS XE, against a real 1400-byte link. The 28-byte math and the PMTUD black holes that hang TCP.
Ping
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Cisco Extended Ping and Response Characters Explained
Source interfaces, the DF bit, repeat counts, and the response characters (! . U M) that tell you exactly what failed. Extended ping on Cisco IOS XE, captured live.
Ping
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Ping Timeout vs Destination Unreachable: Decoding Every Failure
Silent 100% loss and Destination Unreachable are different failures pointing at different problems. Every ping failure message decoded with live lab captures.
Ping
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How Ping Works: The ICMP Echo Exchange Explained
What actually happens between typing ping and the reply line, decoded field by field on a live 3-hop Cisco lab: ICMP echo, TTL, RTT, and jitter.
OSPF
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OSPF Stub Area Configuration: Stub, Totally Stubby and NSSA, Proved in the LSDB
One internal router, one area, stepped through stub, totally stubby and NSSA on CML IOS XE 17.18.2. The LSDB and routing table at every stage show which LSAs survive, which routes vanish, and the Type-7 to Type-5 translation proved on the far side of the ABR.
OSPF
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OSPF MTU Mismatch: Neighbor Stuck in EXSTART, and Both Fixes
A neighbor parked in EXSTART means the DBD exchange is failing, and IP MTU mismatch is almost always why. Captured on CML with IOS XE 17.18.2: the asymmetric stuck states, the debug line that names the cause, a recipe to reproduce it, and both fixes proven all the way to FULL.