Default routes and the black holes they create
A default route means "send it here and hope". Sometimes hope is misplaced.
Writing
Troubleshooting
When OSPF neighbours will not form
The adjacency state tells you which check failed. Work the list in order.
OSPF cost and the traffic taking the wrong path
Traffic prefers a slow link over a fast one. The cause is usually a default reference bandwidth.
Interface counters and what they actually tell you
Every error counter has a specific meaning. Knowing which is which turns guessing into diagnosis.
Cable faults that look like software faults
Hours get spent in configuration for problems that live in the first metre of the path.
When traceroute lies
Asterisks are not failures and high latency at hop four is usually nothing. Read it correctly.
TTL as a diagnostic instrument
The time-to-live field quietly tells you how many hops a packet took. That is more useful than it sounds.
Reading a packet capture without drowning
A capture with two million frames is not evidence until you have reduced it. Here is the order that works.
The difference between a slow network and a busy one
Both feel identical to users. They have different causes and different fixes.
ARP, and the day two devices claimed one address
Intermittent, symmetrical, affects everyone on the subnet. Duplicate addresses have a signature.