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Ethernet LAN Fundamentals: Cabling, Framing, and Switch Basics

Ethernet is the dominant technology connecting devices within local networks worldwide, and understanding its physical cabling, frame structure, and switching behavior is foundational to nearly every networking task. This comprehensive guide covers Ethernet cabling standards, the structure of an Ethernet frame, how switches learn and forward traffic using MAC addresses, and the essential commands for viewing and managing a Cisco switch.

Ethernet LANMAC Address SwitchingCisco Switch Commands

~5 دقیقه مطالعه · آخرین به‌روزرسانی ۱۸ شهریور ۱۴۰۵

Ethernet Cabling: Connecting the Physical Layer

Ethernet networks are physically connected using either copper twisted-pair cabling or fiber-optic cabling, each suited to different distances and speeds.

Common Ethernet cabling standards:
UTP (Unshielded Twisted Pair) - most common,
  Cat5e supports 1 Gbps up to 100 meters
  Cat6/6a supports 10 Gbps up to 100 meters (Cat6a)

Fiber Optic - used for longer distances and higher speeds
  Single-mode: long distances (kilometers), used for WAN links
  Multi-mode: shorter distances, common within data centers

Two cable wiring types matter for connecting devices correctly: a Straight-Through Cable connects dissimilar devices (a PC to a switch, or a switch to a router), while a Crossover Cable traditionally connected similar devices (switch to switch). Modern Ethernet ports largely eliminate this distinction through Auto-MDIX, a feature that automatically detects and adjusts for whichever cable type is plugged in.

The Structure of an Ethernet Frame

Data traveling across an Ethernet LAN is organized into Frames, the Layer 2 unit of data discussed earlier in this series regarding encapsulation.

Ethernet II frame structure:

| Preamble | Dest MAC | Src MAC | Type | Data | FCS |
| 8 bytes  | 6 bytes  | 6 bytes | 2B   | 46-1500B | 4B |

Preamble:    synchronizes the receiving device's clock
Dest MAC:    the physical address of the intended recipient
Src MAC:     the physical address of the sender
Type:        identifies the Layer 3 protocol carried inside
              (e.g., 0x0800 for IPv4)
Data:        the actual payload (an IP packet, typically)
FCS:         Frame Check Sequence, used to detect transmission errors

The MAC (Media Access Control) Address, a 48-bit identifier typically written as six pairs of hexadecimal digits, uniquely identifies each network interface at the hardware level. Unlike IP addresses, which are assigned based on network topology, MAC addresses are burned into the hardware by the manufacturer and remain fixed regardless of where the device connects.

How a Switch Learns and Forwards Traffic

A Cisco switch's core job is deciding which port to send an incoming frame out of, and it accomplishes this through a continuously updated MAC Address Table.

Switch forwarding logic:

1. Learn: when a frame arrives on a port, the switch
   records the source MAC address and the port it
   arrived on in its MAC address table

2. Forward/Flood: the switch checks the destination
   MAC address against its table
   - If found: forward the frame only out that specific port
   - If not found: flood the frame out every port
     except the one it arrived on

3. Filter: if the source and destination MAC are on
   the same port's segment, the switch does not
   forward the frame at all

This learning process happens automatically and continuously, without any configuration required, which is why a switch functions correctly the moment it is powered on and cabled — though verifying its learned state is a routine and essential troubleshooting step.

Viewing the MAC Address Table

Switch# show mac address-table

          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0050.56aa.1122    DYNAMIC     Gi1/0/1
   1    0050.56bb.3344    DYNAMIC     Gi1/0/2

This output directly reflects the forwarding logic described above: it shows exactly which MAC address the switch has learned on which port, and is often the very first command run when troubleshooting why a device cannot communicate on the local network.

Full-Duplex Versus Half-Duplex Operation

Modern Ethernet operates in Full-Duplex mode, where a device can send and receive simultaneously on separate physical wire pairs, effectively eliminating collisions entirely. Older or misconfigured connections sometimes fall back to Half-Duplex, where a device can only send or receive at one time, requiring a collision-detection mechanism.

Checking and setting duplex/speed on an interface:

Switch(config)# interface gigabitethernet 1/0/1
Switch(config-if)# duplex full
Switch(config-if)# speed 1000

Switch# show interfaces gigabitethernet 1/0/1
GigabitEthernet1/0/1 is up, line protocol is up
  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX

A Duplex Mismatch, where one end of a link is set to full-duplex and the other to half-duplex, is a classic and notoriously confusing networking problem: the link appears to work, but suffers from unexplained slowness and intermittent errors, since one side expects collision handling that the other side never performs.

Essential Cisco Switch Management Commands

Beyond understanding switching theory, day-to-day work requires fluency with the basic commands for navigating and inspecting a Cisco device.

Switch> enable
Switch# configure terminal
Switch(config)# hostname SW1
SW1(config)# interface vlan 1
SW1(config-if)# ip address 192.168.1.2 255.255.255.0
SW1(config-if)# no shutdown
SW1(config-if)# exit
SW1(config)# exit
SW1# copy running-config startup-config

SW1# show version
SW1# show interfaces status
SW1# show running-config

These commands illustrate the Cisco IOS command hierarchy: enable moves from user mode to privileged mode, configure terminal enters global configuration mode, and interface-specific commands like interface vlan 1 drop into a more specific configuration context. Saving the configuration with copy running-config startup-config is essential — changes made in the running configuration are lost on reboot unless explicitly saved to the startup configuration.

Why These Fundamentals Matter for Everything That Follows

Every more advanced Layer 2 topic covered later in this series — VLANs, Spanning Tree Protocol, and EtherChannel — builds directly on the MAC-address-based learning and forwarding behavior described in this article. Understanding exactly how a switch decides where to send a frame, and being fluent with the basic show commands used to verify that behavior, is the essential foundation for diagnosing the vast majority of real-world local network connectivity problems.

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