Skip to content

Latest commit

 

History

History
121 lines (70 loc) · 3.44 KB

Ethernet_LAN_Switching_Part1.md

File metadata and controls

121 lines (70 loc) · 3.44 KB
Raw

5. ETHERNET LAN SWITCHING : PART 1

image

image

LAN's

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

image

An ETHERNET FRAME looks like:

image

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)

image

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

image

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.

image

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.

image

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