112 lines
3.4 KiB
Markdown
112 lines
3.4 KiB
Markdown
---
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id: 1772845705-JEBL
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aliases:
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- Ethernet LAN Switching
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tags: []
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---
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# Ethernet LAN Switching
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## [[OSI model]] - physical layer
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- Defines physical characteristics of the medium used to transfer data between devices
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- For example, [[voltage]] levels, maximum transmission distances, physical connectors, cable specifications, etc..
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- Digital [[bits]] converted into electrical (for wired connections) or radio (for wireless connections) signals
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- All of the information in day 2 [[02. Interfaces and Cables]] (cables, pin layout, etc.) is related to the Physical layer.
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## OSI model - Data Link Layer
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- Provides node-to-node connectivity and data transfer (for example, PC to switch, switch to router, router to router)
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- Defines how data is formatted for transmission over a physical medium (for example, copper UTP cables)
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- Detects and (possibly) corrects Physical Layer errors.
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- Uses Layer 2 addressing separate from Layer 3 addressing
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- Switches operate at Layer 2.
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## OSI Models - PDUs
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Layer 4 header - Data
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Layer 3 header - Segment
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Layer 2 header - Packet
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Layer 1 header - Frame
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Protocol Data Units | PDUs
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## Ethernet Frame
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ethernet header | Packet | Ethernet Trailer
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### Ehternet Header
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Preamble | Start Frame Delimiter (SFD) | Destination | Source | Type or Length
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7 bytes 1 byte 6 byte 6 byte 2 bytes
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#### Preamble
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- Length : 7 Bytes (56 bits)
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- Alternating 1's and 0's
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- 10101010 * 7
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- Allows devices to synchronize their receiver clocks
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#### SFD
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- Start Frame Delimiter
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- Length: 1 byte (8 bits)
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- 10101011
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- Marks the end of the preamble, and the beginning of the rest of the frame
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#### Destination & Source
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Indicate the devices sending and receiving the frame
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- Consist of the desination and source [[MAC Address]]
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- MAC = Media Access Constol
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- = 6 byte (48-bit) address of the physical device
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#### Type / Length
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- 2 byte (16-bit) field
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- A value of 1500 or less in this field indicate the LENGTH of the encapsulated packet (in bytes)
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- A value of 1536 or greater in this field indicates the TYPE of the encapsulated packet (Usually [[IPv4]] or [[IPv6]])
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and the length is determined via other methods
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IPv4 = 0x0800 ([[hexadecimal]])
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(2048 in decimal)
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IPv6 = 0x86DD (hexadecimal)
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(34525 in decimal)
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### Ethernet Trailer
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Frame Check Sequence (FCS)
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4 bytes
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#### Frame Check Sequence
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- 4 bytes (32 bits) in length
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- Detects corrupted data by running a 'CRC' algorithm over the received data
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- CRC = Cycling Redundancy Check
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## Mac Address
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- 6-byte (48-bit) physical address assigned to the device when it is made
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- AKA ' Burned-in Address' (BIA)
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- Is globally unique
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- The first 3 bytes are the OUI (Organizationally Unique Identifier), which is assigned to the company making the device
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- The last 3 bytes are unique to the device itself
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- Written as 12 hexadecimal character
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Unicast frame: a frame destined for a single target
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### Dynamically learned Mac Address
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Is a mac address learned by the switche automatically by looking at the frame Destination Frame
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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)
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until the PC replied to another pc
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**Unknown Unicast Frame** -> *Flood*
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if the Mac address is knowned by the switch it will directly send it in the interface
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**Known Unicast Frame** -> *Forward*
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*Dynamically Mac Addresses are removed from the MAC address table after 5 minutes of inactivity*
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