<Jeremy's IT Lab> Day5: Ethernet LAN Switching (Part1)

Ethernet LAN Switching (Part1)

 

PDUs; frame

 

*preamble 과 SFD 영역은 Header로 생각되지 않음.

*Minimum size for an Ethernet frame: 64 bytess

 

■ Header (8 bytes)

Preamble (7 bytes)

-> synchronize device's receiver clocks

-> 10101010 * 7

 

SFD (1 bytes)

->Marks the end of preamble, beginning of rest of the frame

-> 10101011

 

Destination & Source (6 bytes)

-> MAC address

 

Type(Length) (2 bytes)

-> 1500 or less, indicates LENGTH of encapsulated packet

-> 1536 or greater, indicates TYPE of encapsulated packet(IPv4 or IPv6)

IPv4 = 0x0800 //2048 in decimal

IPv6 = 0x86DD //34525 in decimal

 

 

■ Trailer (4 bytes)

FCD (4 bytes)

-> detects corrupted data by running 'CRC' algorithm

 

=> total 26 bytes

=> Header + trailer: 18 bytes

 

■ Packet (Payload) (over 46 bytes)

-> payload less than 46 bytes, padding bytes are added

-> ei. 36 bytes packet + 10 bytes padding bytes = 46 bytes

 

Switching

MAC

-> A.K.A. 'Burned-In Address' (BIA)

-> globally unique

-> first 3 bytes are OUI, assigned to the company

->last 3 bytes are unique to device itself

 

MAC Address Table:

Dynamically learned MAC address / Dynamic MAC Address

-> addresses removed after 5 mins (Aging)

 

Unicast frame: a frame destined for a single target

Unknown Unicast frame = FLOOD

Known Unicast frame = FORWARD

 

Case1.

PC1 transmit data to PC3

Dest: .0003

Src: .0001

* FO/1 : fast 100mbit/sec

Process of Transmitting Data

(1) PC1 send frame to SW1

(2) Fill address into SW1 table

(3) Flood frame to PC2, SW2 (PC2 drop frame)

(4) Fill PC1 address into SW2 table

(5) Flood frame to PC3, PC4 (PC4 drop frame)

(6) PC3 replies back to SW2

(7) Fill PC3 address into SW2 table

(8) SW2 forwards frame to SW1

(9) Fill PC3 address to SW1

(10) SW1 forwards frame to PC1

 

 

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Acronyms

• SFD : Start Frame Delimiter
• FCS : Frame Check Sequence
• CRE : Cyclic Redundancy Check
• OUI : Organizationally Unique Identifier