STP, RSTP and MSTP

What this topic solves

Loop prevention, root election, port roles/states, rapid convergence, MST regions and Layer 2 protections.

Follow state and packet path

Validate the Layer 2 domain, VLAN/bridge-domain, MAC learning, port state, loops, MTU and counters before changing configuration.

Input → classification/state → decision → table → next-hop/output → counters

Design before configuration

• Define the failure domain and expected convergence behavior.
• Document addressing, identifiers, timers, MTU and policy boundaries.
• Prefer explicit, observable state over assumptions.
• Plan rollback and out-of-band access before risky changes.

Commands and verification

Command syntax varies by platform and software release. The examples below come from the existing portal content and must be validated against the target platform documentation before production use.

SW1 priority 4096   → candidato preferido
SW2 priority 8192
SW3 priority 32768

Objetivo: posicionar root e root secundário
de acordo com o desenho do domínio L2.

Região MST "CAMPUS"
Revisão: 10

MSTI 1 → VLANs 10,20,30
MSTI 2 → VLANs 40,50,60

Objetivo possível:
MSTI 1 prefere CORE-A
MSTI 2 prefere CORE-B

Evidence-driven workflow

1. Define source, destination, time window and exact symptom.
2. Establish the expected healthy state and compare it with the observed state.
3. Inspect counters, tables, logs and packet captures before resets.
4. Test one hypothesis at a time and prefer reversible changes.
5. Repeat the original test after correction and check side effects.

IEEE 802.1Q / RSTP / MSTP

Use the current standard/RFC and the documentation for the exact software release being operated. Vendor syntax is an implementation detail; protocol behavior should be understood independently.

Prove the behavior in a controlled topology

Build the smallest topology that reproduces the mechanism, capture control and data traffic, introduce one deliberate failure and diagnose it before applying the fix.

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