CCNA Notes

200-301 Exam Prep

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1Networking Devices2Interfaces And Cables3OSI Model and TCP/IP Suite4Intro to the CLI5Ethernet LAN Switching - Part 16Ethernet LAN Switching - Part 27IPv4 Addressing - Part 18IPv4 Addressing - Part 29Switch Interfaces10The IPv4 Header11Routing Fundamentals - Part 111Static Routing - Part 212Life of a Packet13Subnetting - Part 114Subnetting - Part 215Subnetting (VLSM) - Part 316VLANs - Part 117VLANs - Part 218VLANs - Part 319DTP / VTP20Spanning Tree Protocol - Part 121Spanning Tree Protocol - Part 222Rapid Spanning Tree Protocol23Etherchannel24Dynamic Routing25RIP / EIGRP26OSPF - Part 127OSPF - Part 228OSPF - Part 329First Hop Redundancy Protocol30TCP and UDP31IPv6 - Part 132IPv6 - Part 233IPv6 - Part 334Standard Access Control List35Extended Access Control List36CDP and LLDP37NTP - Network Time Protocol38DNS - Domain Name System39DHCP - Dynamic Host Configuration Protocol40SNMP - Simple Network Management Protocol41SYSLOG42SSH - Secure Shell43FTP and TFTP44NAT (Static) - Part 145NAT (Dynamic) - Part 246QoS (Voice VLAN) - Part 147QoS (Quality of Service) - Part 248Security Fundamentals49Port Security50DHCP Snooping51Dynamic ARP Inspection52LAN Architectures53WAN Architectures54Virtualization and Cloud - Part 154Virtualization (Containers) - Part 254Virtualization (VRF) - Part 355Wireless Fundamentals56Wireless Architectures57Wireless Security58Wireless Configuration59Introduction to Network Automation60JSON, XML, and YAML61REST APIs62Software Defined Networking63Ansible, Puppet, and Chef67CCNA Complete Cheatsheet
/Ethernet LAN Switching - Part 1
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Ethernet LAN Switching - Part 1

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5. ETHERNET LAN SWITCHING : PART 1

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LAN's

  • A LAN is a network contained in a relatively small area.
  • Routers are used to connect separate LAN's

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An ETHERNET FRAME looks like:

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Ethernet Trailer --- PACKET --- Ethernet Header

The Ethernet Header contains 5 Fields:

Preamble -- SFD -- Destination -- Source -- Type 7 bytes -- 1 byte -- 6 bytes -- 6 bytes -- 2 bytes

PREAMBLE:

  • Length: 7 bytes (56 bits)
  • Alternating 1's and 0's
  • 10101010 * 7x
  • Allows devices to synchronize their receiver clocks

SFD : ‘Start Frame Delimiter’

  • Length: 1 byte(8 bits)
  • 10101011
  • Marks end of the PREAMBLE and beginning of rest of frame.

DESTINATION AND SOURCE

  • Layer 2 Address
  • Indicates the devices sending / receiving the frame
  • MAC = ’Media Access Control’
  • = 6 byte (48-bit) address of the physical device

TYPE / LENGTH

  • 2 bytes (16-bit) field
  • A value of 1500 or less in this field indicates the LENGTH of the encapsulated packet (in bytes)
  • A value of 1536 or greater in this field indicates the TYPE of the encapsulated packet and length is determined via other methods.
  • IPv4 = 0x0800 (hexadecimal) = 2048 in decimal
  • IPv6 = 0x86DD (hexadecimal) = 34525 in decimal
  • Layer 3 protocol used in the encapsulated Packet, which is almost always Internet Protocol (IP) version 4 or version 6.

The ETHERNET TRAILER contains:

FCS

  • ‘FRAME CHECK SEQUENCE’
  • 4 bytes (32 bits) in length
  • Detects corrupted data by running a 'CRC' algorithm over the received data
  • CRC = "Cyclic Redundancy Check"

Altogether the ETHERNET FRAME = 26 bytes (header + trailer)

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MAC ADDRESS (48 bits long)

  • 6-bytes (48-bits) physical address assigned to the device when it is made.
  • AKA 'Burned-In Address' (BIA)
  • Is globally unique
  • First 3 bytes are the OUI (Organizationally Unique Identifier) which is assigned to the company making the device
  • The last 3 bytes are unique to the device itself
  • Written as 12 hexadecimal characters

Example:

E8:BA:70 // 11:28:74 OUI // Unique Device ID

HEXADECIMAL

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INTERFACE NAMES

F0/1, F0/2, F0/3... F stands for "Fast Ethernet" or 100 Mbps interfaces.

MAC ADDRESS TABLE

Each Switch stores a DYNAMICALLY LEARNED MAC ADDRESS TABLE, using the SOURCE MAC ADDRESS of frames it receives.

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When a Switch doesn't know the DESTINATION MAC ADDRESS of a frame (UNKNOWN UNICAST FRAME), it is forced to FLOOD the frame - Forward the frame out of ALL it's interfaces, except the one it received the packet from.

When a KNOWN Unicast Frame is known (MAC Address is recognized by the entry in the MAC ADDRESS TABLE), the frame is FORWARDED like normal.

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  • Note: Dynamic MAC Addresses are removed from the MAC ADDRESS TABLE every 5 minutes of inactivity.
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