This article will clarify the switching concepts because switching and frame forwarding are common in networking and telecommunication. LAN, WAN, and PSTN use different types of switches. The basic switching concept is based on the following two criteria.

• Ingress port
• Destination address

The traffic forwarding decision is completed in relation to the traffic flow. The term ingress explains where a frame enters the device on a particular port. The term egress describes the frames leaving the device from a particular port.

A switch makes a frame forwarding decision based on the ingress port and the destination host’s address.  The networking switch also maintains a MAC address table.

The switch decides from the MAC address table where to send traffic. The figure below illustrates the switch MAC address table.

• If the data centre switches port 1 and the message has a destination address of DB-CD-AC-3D-26-25
• Then, the switch forwards the data out to port 7.
• If the data centre switches port 6 and the message has a destination address of C3-A3-A2-35-A6-66
• Then, the switch forwards the data out of port 8.
• If a data centre switches port 2 with a destination address of EE-01-A1-AF-00-01.
• Then, the switch forwards the data out of port 1.

The network switch has only one intelligence: the ability to keep up the MAC address table and decide to send traffic based on the ingress port and the message’s destination address.

Every switch has only one master table that describes an association between address and port. The Cisco switches forward Ethernet frames based on the destination MAC address of the frames.

The switching concept of Dynamically Populating MAC Address Table

Network Switches use the MAC address table to send network traffic towards the destination through the proper port. It is also necessary for a switch to know which port to use to send data. The switch must first learn which devices exist on each port. When the switch learns the devices connected to each port, it prepares the MAC address.

The table is also called a content addressable memory (CAM) table. The high-speed searching application uses switch CAM memory. The switches decide how to handle incoming data frames by maintaining the table of MAC addresses.

The switch builds a MAC address table by reading the MAC address of each device connected to the port of a switch and then uses the MAC address table to send frames destined for a particular device out of the specific port assigned to that device.

The switch populates the table of the MAC address based on the source MAC addresses. When a switch receives an incoming frame with a destination MAC address if the destination MAC address is not available in the table, the switch forwards the frame out of all ports except for the ingress port of the frame; this technique is called flooding of data.

When the destination device responds, the switch adds the source MAC address of the frame and the port where the frame was just received to the MAC address table. In a network where multiple switches are interconnected, The MAC address table also has multiple addresses for a single port.

The switch kept MAC address for a specific time, the ageing time is five minutes for a single host entry. The steps below describe the process of building the MAC address table.

The following steps will clear the switching concept and build the MAC address table:

• The switch receives a frame from Host 1 on Port 1, and the MAC address table is empty (Figure 1).

• The switch examines the MAC address of the source (Host 1) and compares it to the MAC address table.
• If the address is not found in the MAC address table, it associates the MAC address of host 1 with port 1 (ingress port) in the MAC address table. (Figure 2)

• If the MAC address already exists in the table, the ageing timer for an entry is reset.
• When the switch has recorded the source address information in the MAC table, it examines the destination MAC address.
• If the destination address is not already recorded in the MAC table or a broadcast MAC address containing all Fs, the switch floods the data frame to all ports except the ingress port (Figure 3).

• The destination Host (Host 4) will reply to the frame with a unicast frame addressed to Host 1 (Figure 4). The switch enters the source MAC address of Host 4 and the port number of the ingress port into the address table. The MAC address table shows the frame’s destination address (Host 1) and associated egress ports.

• The switch now completes entries for host 1 and host 4, and they can now send frames between these source and destination devices without flooding.