Chapter 5

🚌 Switch Basics - The Smart Bus Driver

By Sys-Metrics· · 45 min chapter

🎯 Meet the Network's Traffic Director

Switches are the workhorses of modern networks. Like a smart bus driver who remembers where every passenger lives and takes the most efficient routes, switches learn device locations and forward traffic intelligently. Understanding switches is crucial for any network professional.

🎯 Chapter Goals: Understand switch operation, master MAC address learning, configure basic switch settings, and troubleshoot common switching issues!

🚌 What is a Network Switch?

A network switch is a Layer 2 device that forwards Ethernet frames based on MAC addresses. Think of it as an intelligent traffic director for your local network.

Switch vs Hub (The Evolution)

📻 Hub (Obsolete)

  • Operation: Repeats all data to all ports
  • Collision Domain: All ports share one domain
  • Bandwidth: Shared among all devices
  • Intelligence: None - just repeats signals
  • Duplex: Half-duplex only
  • Security: All devices see all traffic

🚌 Switch (Modern)

  • Operation: Learns and forwards intelligently
  • Collision Domain: Each port is separate domain
  • Bandwidth: Dedicated per port
  • Intelligence: Maintains MAC address table
  • Duplex: Full-duplex capable
  • Security: Traffic only to intended recipient

Key Switch Benefits

Collision Domain Separation

Each switch port creates its own collision domain, eliminating collisions

Dedicated Bandwidth

Each port gets full bandwidth (100Mbps, 1Gbps, etc.)

Intelligent Forwarding

Only sends frames to intended recipients, reducing network congestion

Full Duplex

Simultaneous send and receive, effectively doubling throughput

🧠 How Switches Learn - The MAC Address Table

Switches maintain a MAC address table (also called CAM table) to track device locations. This is like a bus driver's mental map of where each passenger lives.

The Learning Process

1
Frame Arrives
Switch receives an Ethernet frame on a port
2
Learn Source MAC
Records source MAC address and incoming port in table
3
Check Destination
Looks up destination MAC in the table
4
Forward Decision
Forwards to specific port or floods if unknown

MAC Address Table Example

MAC Address Port Age (seconds)
00:1B:44:11:3A:B7 Fa0/1 45
00:50:56:C0:00:08 Fa0/5 12
AA:BB:CC:DD:EE:FF Fa0/12 156
🧠 Memory Trick: Switches learn source addresses (where frames come FROM) to know where to send frames TO later.

📦 Frame Forwarding Decisions

When a switch receives a frame, it makes one of three decisions based on the destination MAC address:

1. Forward (Unicast)

Condition

Destination MAC address is in the table and on a different port

Action

Forward frame only to the specific destination port

Benefit

Efficient - doesn't waste bandwidth on other ports

2. Filter (Drop)

Condition

Destination MAC address is in the table on the same port as source

Action

Drop the frame (devices on same port can communicate directly)

Benefit

Prevents unnecessary traffic from leaving the collision domain

3. Flood

Condition

Destination MAC address is unknown, broadcast, or multicast

Action

Send frame to all ports except the source port

Downside

Creates traffic on all segments, but necessary for discovery

Frame Processing Example

Scenario: PC-A (port 1) sends to PC-B (port 3)
📥
Frame arrives on Port 1 with Source: MAC-A, Destination: MAC-B
📝
Switch learns "MAC-A is on Port 1" (if not already known)
🔍
Switch looks up MAC-B in table
📤
If MAC-B on Port 3: Forward to Port 3 only
If MAC-B unknown: Flood to all ports except Port 1

🔄 MAC Address Aging

MAC address table entries don't last forever. Switches use aging to keep tables current and prevent them from filling up.

Aging Process

Default Timer

300 seconds (5 minutes) - configurable

Refresh Mechanism

Every time a frame is received from a MAC, its timer resets to 0

Aging Out

If no frames from a MAC for 300 seconds, entry is removed

Why Necessary

Devices move, get unplugged, or change ports - table must stay accurate

MAC Table Management Commands

Switch# show mac address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0050.56c0.0008 DYNAMIC Fa0/5
1 001b.4411.3ab7 DYNAMIC Fa0/1
Total Mac Addresses for this criterion: 2
Switch# clear mac address-table dynamic
% All dynamic entries will be deleted

⚙️ Basic Switch Configuration

Initial Switch Setup

Switch> enable
Switch# configure terminal
Switch(config)# hostname SW1
SW1(config)# enable secret cisco123
SW1(config)# line console 0
SW1(config-line)# password console123
SW1(config-line)# login
SW1(config-line)# exit

Management IP Configuration

SW1(config)# interface vlan 1
SW1(config-if)# ip address 192.168.1.10 255.255.255.0
SW1(config-if)# no shutdown
SW1(config-if)# exit
SW1(config)# ip default-gateway 192.168.1.1
SW1(config)# end
SW1# copy running-config startup-config

Interface Configuration

interface fastethernet 0/1

Enter interface configuration mode for a specific port

description "Server Connection"

Add descriptive text to identify the interface purpose

speed 100

Force interface speed (10, 100, 1000, or auto)

duplex full

Set duplex mode (half, full, or auto)

shutdown

Administratively disable the port

no shutdown

Enable the port (default state)

🔍 Switch Monitoring & Verification

Essential Show Commands

show mac address-table

Display learned MAC addresses and their ports

show interfaces

Detailed statistics for all interfaces

show interfaces fastethernet 0/1

Detailed stats for specific interface

show ip interface brief

Quick summary of all interfaces

show interfaces status

Port status, VLAN, duplex, speed information

show version

Switch model, IOS version, uptime, memory

Interface Status Indicators

Up/Up
Physical and logical link working
Up/Down
Physical OK, but no Layer 2 protocol
Down/Down
No physical connection detected
Admin Down
Port disabled by shutdown command

Interface Statistics to Monitor

Input/Output Packets

Total frames processed - should be increasing with activity

Input/Output Errors

Should be 0 or very low - high errors indicate problems

CRC Errors

Frames with checksum errors - indicates bad cables or EMI

Collisions

Should be 0 on modern full-duplex links

Runts/Giants

Frames too small/large - indicates configuration issues

🚨 Common Switch Problems & Solutions

Port Issues

❌ Port Shows Down/Down
Physical layer problem
✅ Troubleshooting Steps:
1. Check cable connections (both ends)
2. Try different cable
3. Check if port is shutdown
4. Verify device on other end is powered
❌ Slow Network Performance
Users complaining about speed
✅ Check These Issues:
1. Duplex mismatch (half vs full)
2. Speed mismatch (10/100/1000)
3. High error rates on interfaces
4. Broadcast storms or loops
❌ Intermittent Connectivity
Connection works sometimes
✅ Potential Causes:
1. Loose cable connections
2. Overheating switch
3. Power supply issues
4. MAC address flapping

Duplex Mismatch Problems

One of the most common and hardest to diagnose switch issues:

Symptom

Slow performance, high collision counts, intermittent connectivity

Cause

One end configured for half-duplex, other end for full-duplex

Detection

show interfaces - look for collisions on full-duplex interfaces

Solution

Configure both ends the same: duplex auto or manually match settings

🔒 Port Security Basics

Port security allows you to control which devices can connect to switch ports, preventing unauthorized access.

Port Security Features

MAC Address Limiting

Restrict how many MAC addresses can be learned on a port

Static MAC Assignment

Manually specify which MAC addresses are allowed

Violation Actions

Define what happens when security is violated (shutdown, restrict, protect)

Basic Port Security Configuration

SW1(config)# interface fastethernet 0/5
SW1(config-if)# switchport mode access
SW1(config-if)# switchport port-security
SW1(config-if)# switchport port-security maximum 2
SW1(config-if)# switchport port-security violation shutdown
SW1(config-if)# switchport port-security mac-address sticky

Violation Actions Explained

Shutdown (Default)

Disables the port when violation occurs - most secure

Restrict

Drops violating frames but keeps port up - logs violations

Protect

Drops violating frames silently - no logging

🔄 Broadcast Domains vs Collision Domains

Understanding these concepts is crucial for network design and troubleshooting.

Collision Domains

Definition

Network segment where collisions can occur when two devices transmit simultaneously

Hub Behavior

All ports share one collision domain

Switch Behavior

Each port creates separate collision domain

Modern Reality

Full-duplex eliminates collisions entirely

Broadcast Domains

Definition

Network segment where broadcast frames are propagated

Switch Behavior

All ports in same VLAN share broadcast domain

Router Behavior

Routers separate broadcast domains

Impact

Large broadcast domains can cause performance issues

Domain Comparison Example

24-port switch with all ports in default VLAN:
24
Collision Domains
Each port separate
1
Broadcast Domain
All ports together

🛠️ Hands-On Labs

Lab 1: MAC Address Learning Observation

  1. Setup: Connect 3 PCs to a switch in Packet Tracer
  2. Baseline: Check MAC table: show mac address-table
  3. Generate Traffic:
    • PC1 ping PC2
    • PC2 ping PC3
    • Check MAC table after each ping
  4. Observe: Watch how switch learns MAC addresses
  5. Clear Table: clear mac address-table dynamic
  6. Repeat: Generate traffic again and watch relearning

Lab 2: Basic Switch Configuration

  1. Access Console: Connect to switch console
  2. Basic Setup:
    • Set hostname to "Lab-SW1"
    • Configure enable password
    • Set console password
  3. Management IP:
    • Configure VLAN 1 with IP 192.168.1.50/24
    • Set default gateway 192.168.1.1
    • Test connectivity with ping
  4. Port Configuration:
    • Add descriptions to active ports
    • Practice shutdown/no shutdown

Lab 3: Interface Monitoring

  1. Monitor Commands:
    • show interfaces status - port summary
    • show ip interface brief - quick overview
    • show interfaces fa0/1 - detailed port stats
  2. Generate Traffic: Ping between connected devices
  3. Watch Counters: Run show commands again, observe changes
  4. Error Simulation:
    • Unplug cable, check status
    • Shutdown port, observe admin down status

Lab 4: Troubleshooting Challenge

Scenario: Create these problems and practice fixing them:

  • Shutdown a port and diagnose "no connectivity"
  • Force speed mismatch and observe performance impact
  • Configure duplex mismatch and identify symptoms
  • Use wrong cable type and troubleshoot
🎯 Pro Challenge: Set up a network where PC-A can ping PC-B but not PC-C. Use switch configuration and observation to determine why.

📖 Chapter Summary

  • Switch Function: Layer 2 device that forwards based on MAC addresses
  • Learning Process: Maintains MAC table by learning source addresses
  • Forwarding Decisions: Forward, filter, or flood based on destination MAC
  • Collision Domains: Each port creates separate collision domain
  • Full Duplex: Simultaneous send/receive eliminates collisions
  • Configuration: Hostname, passwords, management IP, interface settings
  • Monitoring: MAC table, interface stats, status verification
  • Troubleshooting: Common issues like duplex mismatches, cable problems
🎯 Switching Mastery! You now understand how modern Ethernet networks operate at Layer 2. Switches are the foundation of every LAN.

📝 Switching Fundamentals Quiz

1. How does a switch learn MAC addresses? By examining source MAC addresses of incoming frames and associating them with the receiving port

2. What happens when a switch receives a frame with unknown destination MAC? It floods the frame to all ports except the source port

3. What's the default MAC address aging time? 300 seconds (5 minutes)

4. What command shows the switch MAC address table? show mac address-table

5. What does "Up/Down" interface status mean? Physical layer is up but no Layer 2 protocol communication

6. What's a common cause of slow performance on switches? Duplex mismatch - one side half-duplex, other side full-duplex

7. How many collision domains does a 24-port switch create? 24 collision domains - each port is a separate collision domain

8. What's the difference between a hub and switch? Hubs share bandwidth and collision domains; switches provide dedicated bandwidth and separate collision domains per port

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