How Switch & MAC Address works in LAN

How Switch & MAC Address works in LAN

This article will give you a brief about component of switch networks and How Switch & MAC Address works in LAN. First we will learn about the basic devices of networks and then we will study the working of Cisco LAN switch and MAC address in detail. This article is very helpful for CCNA students.
The essential hardware for network connectivity is included in the list below. First we will review some devices that are no longer in use and the most important ones, such as switches and routers, we will talk later.

• Repeaters: those that extend the distance so that network traffic can be transferred on 
• a particular type of transmission medium.
• Hubs: which are used to connect nodes to each other when using a star topology, such as 100Base-T
• Switches: those that form fast point-to-point connections for all devices connected to them.
• Bridges: which are intelligent repeaters that send traffic from one segment to another only when the traffic is destined for that other segment.
• Routers: which can intelligently route network traffic in many ways.
• Gateways: these are specific application interfaces that link the seven layers of the OSI model when they are different at one or all levels
• Short-range modems for small connections between buildings

WHAT IS A SWITCH?

An Ethernet switch is a layer 2 device that creates a MAC address table to make a
forwarding decision for each frame. Sometimes, the MAC address table is called "addressable content memory table" (CAM)

Switches, as the name implies, can switch connections from one port to another and very quickly. They are connection oriented and dynamically switch between their different ports to create these connections. The interesting thing is that, because connections from one port to another on the switch are carried out as needed (and not sent to ports that are not involved in traffic), the switches can help
eliminate collisions from traffic caused by segments that do not communicate. Unlike the Ethernet hubs that repeat the bits through all ports except the input, an Ethernet switch consults a MAC address table to make a forwarding decision for each frame.

HOW SWITCH FINDS MAC ADDRESSES?

The switches dynamically elaborate the CAM by monitoring the source MAC addresses. The switch forwards frames if it finds a match between the destination MAC address of the frame and an entry in the MAC address table. There are two processes that are performed for each Ethernet frame that enters a switch:

EXAMINE THE SOURCE MAC ADDRESS

Each frame that enters a switch is checked for new addresses. This is done by examining the source MAC address of the frame and the port number through which it entered the switch.

• If the source MAC address does not exist, it is added to the table, along with the input port number.
• If the source MAC address exists, the switch updates the update timer for that entry. By default, most Ethernet switches save an entry in the table for five minutes .

EXAMINE THE DESTINATION MAC ADDRESS

If the destination MAC address is a unicast address, the switch searches for a match between the destination MAC address of the frame and an entry in the MAC address table.

• If the destination MAC address is in the table, it resends the frame through the specified port.
• If the destination MAC address is not in the table, the switch forwards the frame through all ports, except the input. This is known as "unknown unicast."

If the destination MAC address is broadcast or multicast, the frame is also sent over all ports, except the incoming one.

SWITCH MAC ADDRESS TABLE

For a better understanding let's see the following image and the following steps:

• The switch receives a broadcast frame from PC 0 on port 0.
• The switch enters the source MAC address and the switch port that received the frame in the address table.
• Since the destination address is broadcast, the switch saturates all ports by sending the frame, except the port that received it.
• The destination device responds to the broadcast with a unicast frame addressed to PC 0
• The switch enters the source MAC address of PC 1 and the number of the switch port that received the frame into the address table.
• Now the switch can send frames between the source and destination devices without saturating the traffic, since it has entries in the address table that identify the associated ports.
• When a switch sends traffic to a destination MAC address that is not in its MAC table, it will be forwarded to all ports except the source port.

FRAME FORWARDING METHODS OF CISCO SWITCHES

The switches use one of the following forwarding methods for switching data between ports on the network:

• Storage Switching and forwarding:  Receive the full frame and calculate the Cyclic Redundancy Check (CRC). If the CRC is valid, the switch searches for the destination address and sends the frame through the correct port.
• The CRC uses a mathematical formula based on the number of bits (numbers one) to determine if it has an error.
• Cutting Method Switching : The switch does not perform any verification of errors in the frame and has two variants:
• Fast forwarding  (typical): The lowest latency level resends the packet immediately after reading the destination address.
• Fragment-free : The switch stores the first 64 bytes of the frame before forwarding. Most network errors and collisions occur in those first 64 bytes.
• Buffering memory on switches : Use this technique to store frames before sending them or when the destination port is busy due to congestion.

There are two methods of memory buffering: port-based memory and shared memory.

• Port-based memory buffer : frames are stored in queues connected to specific input and output ports
• Shared memory buffer : deposits all frames in a common memory buffer shared by all switch ports.

For asymmetric switching (ports with different speeds), shared memory buffering is optimal because the number of frames stored in the buffer is limited only by the size of the memory buffer as a whole.

CONFIGURING THE PORTS OF A SWITCH

Two of the most basic parameters of a switch are bandwidth and duplex parameters for each individual switch port. It is essential both coincide between the switch port and the connected devices, such as a computer or other switch.

There are two types of duplex parameters used for communications in an Ethernet network:

• Full duplex : both ends of the connection can send and receive data simultaneously.
• Medium duplex : only one end of the connection can send data at a time.

AUTO-MDX

This feature helps reduce configuration errors since the switch detects the type of cable connected to the port and configures the interfaces appropriately