Spanning tree: what it protects you from
A layer 2 loop does not degrade a network. It stops it, completely, in seconds.
Platform
Cisco IOS / IOS-XE
Cisco routers and switches running IOS or IOS-XE. Much of it carries to NX-OS.
ARP, and the day two devices claimed one address
Intermittent, symmetrical, affects everyone on the subnet. Duplicate addresses have a signature.
Duplex mismatch: the fault that hides in plain sight
The link is up, the speed is right, and throughput is terrible. Check the counters, not the status.
Reading a routing table you did not build
Somebody else built it, nobody documented it, and it is now yours. Here is the order to read it in.