PFC and ECN are not configuration checkboxes. They are active mechanisms with specific timing, thresholds, and interactions that can fail in subtle ways.
You know how PFC works at the wire level -- the pause frame anatomy, pause quanta timing at 400G, the headroom calculation, and the three misconfiguration patterns. You know how pause storms form, why circular pause dependencies are possible in mesh fabrics, and why the watchdog is not optional.
You know what ECN actually does -- CE bit marking, the RED algorithm, ECN thresholds and their relationship to PFC headroom. You know DCQCN as the NIC-firmware rate control algorithm that responds to ECN marks, and the counters that tell you whether it is working.
You know the congestion timeline showing how ECN handles mild congestion before PFC is needed, and how to distinguish a configuration failure from a capacity failure.
You know the complete RoCEv2 port configuration checklist: MTU 9000, DSCP trust, DSCP-to-CoS mapping, ETS strict priority, PFC on CoS 3, ECN thresholds. And you know why show roce showing six correct values is the final verification.
Lab 2 is waiting. The buffer utilisation is at 87%. There are no drops. ECN is not configured. You know exactly what to look for.