Spanning Tree Protocols and Loop Prevention
Layer 2 loops cause broadcast storms and MAC flapping. STP builds a loop-free tree via root election (lowest bridge ID = priority + MAC), root port selection (lowest path cost to root), and designated port election per segment. Know 802.1D (classic), 802.1w (RSTP-fast convergence via proposal/agreement), and 802.1s (MSTP-multiple instances, per-VLAN load balancing). Port states: blocking, listening, learning, forwarding (classic); discarding/learning/forwarding (RSTP).
Traps: (1) Lower priority wins root-default 32768; set root priority low deliberately. (2) Path cost is cumulative; tie-break by lowest sender bridge ID then port ID. (3) RSTP edge (PortFast) ports skip listening-never enable toward another switch. (4) BPDU Guard shuts edge ports receiving BPDUs; Root Guard prevents downstream root takeover. (5) MSTP requires identical region name, revision, and VLAN-to-instance mapping on all members.
Rule of thumb: explicitly configure root primary/secondary on core switches; never rely on default priority.
Worked example: Two core switches, both priority 32768. The one with the lower MAC becomes root-unpredictable. Fix: set Core-A priority 4096 (root primary) and Core-B 8192 (secondary). Now topology is deterministic and failover is planned.