STP design is about control: who is root, and how traffic flows.

Design choice	Why
Fast Root Bridge (root primary)	Predictable traffic
Secondary root	Fast failover
Per-VLAN roots	Load balancing
MST mapping	Scalability

Exam memory rule

Root is placed – not picked at random

Traps ⚠️

• No secondary root
• Default priority everywhere
• Wrong VLAN mapping in MST