Spanning Tree and Link Aggregation
Spanning Tree Protocol prevents Layer 2 loops by electing a root bridge and blocking redundant paths. Know the election: lowest bridge ID wins, where bridge ID = priority (default 32768, in 4096 steps) + MAC. Then each non-root switch picks a root port (lowest cost to root), each segment picks a designated port, and everything else is blocked. Port states in classic 802.1D: blocking, listening, learning, forwarding, disabled. RSTP (802.1w) collapses these to discarding, learning, forwarding and converges in seconds. MSTP (802.1s) maps VLANs to instances so you can load-balance.
Link aggregation (LACP, 802.3ad) bundles links into a logical channel. The rules that trip candidates: member links must match in speed, duplex, and VLAN configuration; LACP uses active/passive modes and needs at least one side active; and a misconfigured member is suspended, not silently used.
Traps: forgetting that changing STP priority takes effect immediately but the root may not move if another switch has a lower ID; assuming a blocked port forwards user traffic (it doesn't - it only hears BPDUs); and thinking a LAG provides more bandwidth to a single flow (it doesn't - hashing spreads flows, not packets within a flow).
Rule of thumb: lower priority = more likely to be root. Worked example: Switch A priority 32768, Switch B priority 4096. B is root even if A has more links, because priority is compared before MAC. To force A to be root, set its priority below 4096 (e.g., 0 or 4096 with a lower MAC).