Ethernet Switching

Ethernet is the most common LAN technology. Switches use Ethernet frame information to forward data efficiently between devices on the same local network.

An Ethernet frame contains control information and the data being delivered. Important frame fields include the destination MAC address, source MAC address, type/length information, payload, and error-checking field.

Ethernet uses the Frame Check Sequence (FCS) to help detect damaged frames. If a frame fails the check, it is discarded.

A MAC address is a Layer 2 address assigned to a network interface. It is commonly written as 12 hexadecimal digits, often grouped in pairs.

Example:

00-1A-2B-3C-4D-5E

The first part of a MAC address identifies the organization that assigned the address, while the remaining part identifies the specific interface value assigned by that organization.

MAC addresses are used for local network delivery. They do not replace IP addresses; instead, they work with IP addresses during communication.

An Ethernet broadcast destination MAC address is:

FF-FF-FF-FF-FF-FF

A switch learns MAC addresses by reading the source MAC address of incoming frames. It records which MAC address is reachable through which switch port.

When a switch receives a frame:

1. It learns the source MAC address and incoming port.
2. It checks the destination MAC address.
3. If the destination is known, it forwards the frame only out the correct port.
4. If the destination is unknown, it floods the frame out other ports in the same VLAN.
5. Broadcast frames are flooded to all ports in the broadcast domain except the receiving port.

Switches may use different forwarding methods.

Store-and-forward provides better error checking. Cut-through can reduce latency, but it may forward damaged frames.

Switch interfaces can operate at different speeds, such as 100 Mbps, 1 Gbps, or higher. Modern devices usually negotiate speed and duplex automatically.

Auto-MDIX allows a switch port to adjust automatically for straight-through or crossover cable requirements. This simplifies cabling in many modern networks.