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 2
Topic 63 min read

Ethernet LAN Switching - Part 2

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6. ETHERNET LAN SWITCHING : PART 2

An ETHERNET FRAME looks like:

Ethernet Header --- DATA (Packet) --- Ethernet Trailer

imageClick to enlarge

The Ethernet Header contains 5 Fields:

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

Ethernet Trailer contains 1 Field:

FCS (Frame Check Sequence) = 4 bytes

  • The PREAMBLE + SFD is not usually considered part of the ETHERNET HEADER.

THEREFORE the size of the ETHERNET HEADER + TRAILER is 18 bytes

(6 + 6 + 2 + 4 bytes for the FRAME CHECK SEQUENCE)

The MINIMUM size for an ETHERNET FRAME (Header + Payload [PACKET] + Trailer) is 64 BYTES.

64 BYTES - 18 BYTES (Header + Trailer size) = 46 BYTES

THEREFORE the MINIMUM DATA PAYLOAD (PACKET) size is 46 BYTES!

IF the PAYLOAD is LESS than 46 BYTES then PADDING BYTES are added (padding bytes are a series of 0's) until it equals to 46 BYTES.

When a PC sends a packet to a device with an unknown IP address, it uses an ARP Request.

imageClick to enlarge

  • ARP stands for 'Address Resolution Protocol'.
  • It is used to discover the Layer 2 address (MAC address) of a known Layer 3 address (IP address)
  • Consists of two messages:
    • ARP REQUEST (Source message)
    • ARP REPLY (Destination message)
  • ARP REQUEST is BROADCAST = sent to all hosts on network, except the one it received the request from.

An ARP REQUEST frame has:

  • Source IP Address
  • Destination IP Address
  • Source MAC address
  • BROADCAST MAC Address - FFFF.FFFF.FFFF

An ARP REPLY frame has:

  • Source IP Address
  • Destination IP Address
  • Source MAC address
  • Destination MAC Address

ARP REPLY is a known UNICAST frame = Sent only to the host that sent the ARP REQUEST.

imageClick to enlarge

PING

  • A network utility that is used to test reachability
  • Measures round-trip time
  • Uses two messages:
    • ICMP Echo REQUEST
    • ICMP Echo REPLY
  • Is UNICAST
  • Command to use ping:
    • ping <ip-address>

By Default, a CISCO IOS sends 5 ICMP requests/replies (Default size is 100-bytes)

  • A period (.) is a failed ping
  • An exclamation mark (!) is a successful ping

USEFUL CISCO IOS COMMANDS (from Privileged EXEC mode)

PC1# show arp // shows hosts ARP table

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SW1#show mac address-table // show the switches MAC table

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Will show:

Vlan --- MAC Address --- Type --- Ports(interfaces)

(Vlan = Virtual Local Area Network)

imageClick to enlarge

SW1# clear mac address-table dynamic <optional MAC address>

// clears the entire switches MAC table. // IF the optional MAC address is used, it will clear the SPECFIC MAC address.

SW1 #clear mac address-table dynamic interface <optional Interface>

// clears the MAC table entry of the Switch by it's INTERFACE name.

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