Ethernet LAN Switching (part 1)

OSI model - physical layer

Defines physical characteristics of the medium used to transfer data between devices
For example, voltage levels, maximum transmission distances, physical connectors, cable specifications, etc..
Digital bits converted into electrical (for wired connections) or radio (for wireless connections) signals
All of the information in day 2 02. Interfaces and Cables (cables, pin layout, etc.) is related to the Physical layer.

OSI model - Data Link Layer

Provides node-to-node connectivity and data transfer (for example, PC to switch, switch to router, router to router)
Defines how data is formatted for transmission over a physical medium (for example, copper UTP cables)
Detects and (possibly) corrects Physical Layer errors.
Uses Layer 2 addressing separate from Layer 3 addressing
Switches operate at Layer 2.

OSI Models - PDUs

Layer 4 header - Data Layer 3 header - Segment Layer 2 header - Packet Layer 1 header - Frame

Protocol Data Units | PDUs

Ethernet Frame

ethernet header | Packet | Ethernet Trailer

Ehternet Header

Preamble | Start Frame Delimiter (SFD) | Destination | Source | Type or Length 7 bytes 1 byte 6 byte 6 byte 2 bytes

Preamble

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

SFD

Start Frame Delimiter
Length: 1 byte (8 bits)
10101011
Marks the end of the preamble, and the beginning of the rest of the frame

Destination & Source

Indicate the devices sending and receiving the frame

Consist of the desination and source MAC Address
MAC = Media Access Constol
= 6 byte (48-bit) address of the physical device

Type / Length

2 byte (16-bit) field
A value of 1500 or less in this field indicate the LENGTH of the encapsulated packet (in bytes)
A value of 1536 or greater in this field indicates the TYPE of the encapsulated packet (Usually IPv4 or IPv6) and the length is determined via other methods

IPv4 = 0x0800 (hexadecimal) (2048 in decimal) IPv6 = 0x86DD (hexadecimal) (34525 in decimal)

Ethernet Trailer

Frame Check Sequence (FCS) 4 bytes

Frame Check Sequence

4 bytes (32 bits) in length
Detects corrupted data by running a 'CRC' algorithm over the received data
CRC = Cycling Redundancy Check

Mac Address

6-byte (48-bit) physical address assigned to the device when it is made
AKA ' Burned-in Address' (BIA)
Is globally unique
The 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 character

Unicast frame: a frame destined for a single target

Dynamically learned Mac Address

Is a mac address learned by the switche automatically by looking at the frame Destination Frame

if the Mac address is not in the switch mac address table it will flood the frame (send the frame at all the interfaces except the source mac address) until the PC replied to another pc Unknown Unicast Frame -> Flood

if the Mac address is knowned by the switch it will directly send it in the interface Known Unicast Frame -> Forward

Dynamically Mac Addresses are removed from the MAC address table after 5 minutes of inactivity

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