NETWORKS · FUNDAMENTOS · FUNDAMENTAL

OSI and TCP/IP Models

Layers, responsibilities, PDUs, encapsulation and practical use of reference models during troubleshooting.

01 · OVERVIEW

What this topic solves

Layers, responsibilities, PDUs, encapsulation and practical use of reference models during troubleshooting.

Core concepts: osi, tcp, camadas, pdu, encapsulamento.

02 · MENTAL MODEL

Follow state and packet path

Relate encapsulation, addressing, state and packet path. Separate data, control and management planes before troubleshooting.

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 | EtherType
    [ IPv4 / IPv6 ]
      src IP | dst IP | next-header/protocol
        [ TCP / UDP ]
          src port | dst port
            [ application data ]

tcpdump -eni eth0
tcpdump -ni eth0 arp
tcpdump -ni eth0 icmp
tcpdump -ni eth0 'tcp port 443'
tcpdump -ni eth0 'ip6 or icmp6'

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

Standards and RFCs applicable to the technology

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