Question

The function of a switch is to make intelligent decisions about whether to pass signals on to the next segment of the network. Describe three scenarios in this decision process.

Answer 1

• A switch is a device which operates at data link layer .It is used for connecting the multiple device in the network by using packet switching and uses the MAC address to forward data to the destination.The switch make the intelligent decision about whether to pass the frame signals on to the next segments.
• The switch is called intelligence device because it builds the MAC address table to keep the track of destination address and the port to which these destination address belongs. So switch makes decision by using three scenarios flooding, learning and forwarding These three functions of switch helps in making decisions .

Consider a switch which connects four ports 1,2,3,4 having four workstations A, B, C, D in different segments.These four workstations have their own MAC addresses.The workstations A belongs to port 1, workstations B belong to port 2,workstation C belong to port 3 and workstations D belongs to port 4.

Scenario 1

Flooding

• Suppose that the workstation A sends a frame to workstation B so workstations A frame contain the destination MAC address of B the switch will receive the frame from Workstation A and switch looks in its MAC address table to know which port does the workstations B belongs.
• The switch didn't find any entry of workstation B along with its port number in its MAC address table so the switch initially do the flooding means it will flood the frame of workstations A to all the ports except the port from which it is coming so switch will send the frame to all ports 2,3,4 except port 1 .This is called flooding.

Scenario 2

Learning

• When switch receive the frame from workstations A it learns that frame is coming from port 1 so the switch learn that the workstations A belong to port 1 so it makes a new entry in its MAC address table which contain workstations A MAC address and port number 1.
• The switch floods the frame coming from source workstations A to all the ports 2,3,4 .Now the three workstations will receive the flooded frame and the only workstation B will send a reply to the switch that this destination MAC address in frame belongs to it.
• So now switch also identifies that reply is coming from workstation B which belongs to port 2 so the switch learn that workstation B belongs to port 2 .Now the switch will again add the entry of workstation B MAC address along with its port number 2 .

Scenario 3

Forwarding

• Now the switch has already learnt that workstations A and B belongs to port 1 and port 2 respectively as it also added the entry of MAC  addresses of both the workstations A and B along with their port number 1 and port number 2 so next time when workstations A wants to send a frame to workstation B then switch will directly forward the frame to workstation B. This is called forwarding.
• In this way switch learns the port number of every workstations on the network and make the entry of their MAC address along with their port number in its MAC address table . The switch improves the performance of network by only forwarding the frame to the correct destination .