Basic uses and learning - forwarding mechanism of Layer-2 LAN switch

There are 7 layers in OSI (Open Systems Interconnection) model. The Data-link layer is the 2^nd layer of the OSI model. LAN Switch is the most popular data link layer device. Data link layer address or MAC address (Media Access control) is used for performing switch basic functionalities. LAN switch has some features to alleviate network congestion, one of them is high dense ethernet-ports orientation. Those ports are name ingress and egress ports according to their uses.

Ingress: the ports that are responsible for entering data-frame into LAN switch.

Egress: the ports that are responsible for leaving the data frame from the LAN switch.

Note: Ethernet frame will never be forwarded out using the same port, from where the frame is received.

 

LAN switch is made up of integrated circuit and software that 

can control paths of data-frame. Some fundamental roles of 

switch are:

• receive ethernet frame using ingress ports.

• are responsible for forwarding incoming data-frame by using egress ports.

• responsible for learning (MAC address to port) mapping and maintain MAC address table or CAM (content addressable memory)

 

The data-link layer frame is made up of useful fields, there are: 

Preamble, Destination Mac address, source MAC address, FCS checksum (CRC) etc. These fields are used to operate switch functions. Switches receive incoming ethernet frame, then choose egress ports based on destination MAC address, references table.

The Switch functions can be divided into two steps:

Step 1. Learn – Examining the source MAC Address

Switch checks for new information by investigating source MAC address, and ingress port/incoming port. Then it stores the collected information in its MAC address table.

• If the source MAC the address does not exist in the MAC address table, the MAC address and incoming port number is added to the table.

• If the source MAC address does exist, the switch updates the refresh timer for that entry. By default, most Ethernet switches keep an entry in the table for five minutes. If the source MAC address does exist in the table but on a different port, the switch treats this as a new entry. The entry is replaced using the same MAC address, but with the more current port number.

 
Step 2. Forward - Examining the Destination MAC Address
If the destination MAC the address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table:
·       If the destination MAC the address is in the table, it will forward the frame out of the specified port.
 
·       If the destination MAC address is not in the table, the switch will forward the frame out all ports except the incoming port. This is called an unknown unicast. If the destination MAC address is a broadcast or a multicast, the frame is also flooded out all ports except the incoming port.
Concerning security, latency issues, there are two types of forwarding methods in switch. Layer 2 switches use one of two methods to switch frames:

• Store-and-forward switching - This method makes a forwarding decision on a frame after it has received the entire frame and checked the frame for errors using a mathematical error-checking mechanism known as a cyclic redundancy check (CRC). Store-and-forward switching is Cisco’s primary LAN switching method. it is more distinguishable for its error checking, automatic buffering features.

 

• Cut-through switching - This method begins the forwarding process after the destination MAC address of an incoming frame and the egress port have been determined. So, it is capable of performing rapid frame switching. It has two variants: fast-forward switching, fragment-free switching.

 
Conclusion is that layer-2 switch is better than legacy hub network device and can operate in full duplex mode. It minimizes the collision domain, when it operates in full duplex mode. It maintains MAC address table and switching frame more intelligently. In addition, it has some useful characteristics such as fast port speeds, fast internal switching, large frame buffers, high port density etc. that upgrade network performance by reducing network congestion.