NETWORKS · SWITCHING · FUNDAMENTAL

VLAN and IEEE 802.1Q

Layer 2 segmentation, access and trunk behavior, tagging, PVID/native VLAN, allowed VLANs and troubleshooting.

01 · OVERVIEW

What this topic solves

Layer 2 segmentation, access and trunk behavior, tagging, PVID/native VLAN, allowed VLANs and troubleshooting.

Core concepts: vlan, 802.1q, access, trunk, pvid, native, vlan.

02 · MENTAL MODEL

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

03 · DESIGN

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.

Operational examples

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.

Ethernet:
[Dst MAC][Src MAC][802.1Q tag][EtherType][Payload][FCS]

802.1Q tag:
TPID + PCP + DEI + VID

show vlan brief
show interfaces trunk
show interfaces switchport
show mac address-table
show mac address-table vlan 20
show spanning-tree vlan 20

! Exemplo conceitual
interface GigabitEthernet1/0/10
 switchport mode access
 switchport access vlan 20

interface GigabitEthernet1/0/48
 switchport mode trunk
 switchport trunk allowed vlan 10,20,30,99

04 · TROUBLESHOOTING

Evidence-driven workflow

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

05 · STANDARDS

IEEE 802.1Q

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.

06 · LAB

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.

07 · CONTINUE