LAN Switching

Done

Ethernet Data-link Header

Preamble

It is used to synchronize the receiver`s clock. Also, it is 7 bytes long, each being 10101010

SFD

Indicates that the end of the preamble and the beginning of the rest of the frame. It is 1 byte long, being: 10101011.

Destination

MAC Address (Physical Address) of the device to receive this frame

Source

MAC Address of the device that sent this frame

Type or Lenght

It is 2 bytes long, and if it is 1500 or less, it is a Length field, but if it is 1536 or greater it is a Type field.

Length

Indicates the size of the Packet (L3PDU), which can be 46-1500

Type

Indicates the type of the Packet (L3PDU). Example: 0x0800 indicates a IPv4 Packet

Packet

FCS

By using a complex formula, the result is matched with the sender of the frame, to check if any errors affected the frame.

CRC = Cyclical Redundancy Check

The Preamble+SFD is not considered part of the ethernet header and trailer. Therefore, the size of those are 18 bytes (6+6+2+4).

Unicast Frame

It is a frame destined to a single target.

Unknown Unicast Frame

If the destination`s MAC Address is not known to the switch, in other words, not present in its MAC Address table, it floods the interfaces besides the sender`s one. Therefore, the intended receiver will receive its frame and the other will drop each frame.

Known Unicast Frame

Differently from the occasion above, the destination`s MAC Address is referenced in the MAC Address table of the switch, so it forwards the frame only to the destination`s interface

Dynamically (learned) MAC Address

After receiving a frame, a switch adds the source mac address of the frame to its MAC Address table and associates it with the interface that the frame came from.

The minimun frame’s size is 64 bytes. Therefore, if we subtract the 18 bytes of the header and the trailer, the minimun payload is 46 bytes. So if a packet contais less than 46 bytes, paddings are added with zeros to fill that gap.

ARP (Address Resolution Protocol)

This protocol is a Layer 2 protocol used to discover a device’s MAC address when its IP address (Layer 3) is known.

Consists of:

In the PC, there is what is called an ARP Table. In which the computer remembers a device’s MAC Address and its associated IP Address.

Show MAC Address table CLI

show mac address-table

Clear MAC Address table (Aging)

clear mac address-table dynamic
clear mac address-table dynamic address {MAC ADDRESS}
clear mac address-table dynamic interface {INTERFACE}

Ping

It is an utility to know reachebility thorugh the use of ICMP (Internet Control Message Protocol).