main function of bridge

Ques: What is main function of Bridge?

Ans: Bridge and Switch both work on OSI Reference Model layer 2 (Data Link Layer) and both have same function but Switch is enhance than bridge.

Okay, without going anywhere else we start the main function of Bridge, the function of switch will be discussed in switch function that is slightly different.

As we have discussed that Bridge has main three function.

• Learning MAC Address
• Forwarding Frame and
• Removing loop.

We will see every function one by one. First we will start from Learning MAC address.

Learning MAC Address: In the compare of HUB, Bridge is capable of learning MAC address so that collision domain can be reduced.

• Bridge receives any frame in its port.
• It De-encapsulates the frame and read Source MAC-Address.
• Examine the mac-address-table that mac-address is already exist in table on the same receiving port or not.
  • If mac address is exist,  it updates.
  • If mac address is not exist, it stores the mac-address in mac-address-table with correspondence port no.
• 300 sec is hold down time, It means if any mac-address (available in mac-address table) is in idle state, mac-address will be removed from the table.
• For Learning mac-address, no any special hardware is installed. It is performed by software or some programming.

Forwarding Frame: Ones learning mac-address is done. It is time to forward the frame. But for forwarding the frame destination mac-address is required. Again,

• Bridge receives any frame in its port.
• De-encapsulates the frame.
• Learns the mac-address and updates the mac-address-table.
• Read destination mac-address.
• look at the mac-address table to match the destination mac-address with correspondence port no so that frame can be sent. Yes, You are thinking right. Many possibilities might be :
  • If destination mac-address is found in mac-address-table, called as a “known mac-address”, frame is forwarded to correspondence port no.
  • If destination mac-address is not found in mac-address-table, called as a “unknown mac-address”, frame will be flooded.
  • If destination mac-address is “broadcast address”, frame will be broadcasted.
  • If destination mac-address and source mac-address both found in same port no. Frame will be dropped.

Removing Loop: Redundant link is cause of Bridging loop. The term “loop” means frame or packet continuously moving from one device to another device and again first device and so on. It will keep moving to endless.

• For removing loop many Protocol can be used. such as STP (Spanning Tree Protocol), PVST (Per Vlan Spanning Tree), RSTP (Rapid Spanning Tree Protocol), MST (Multiple Spanning Tree) etc.
• Bridge is only use STP for removing loop. STP finds redundant path and blocks, if any main segment goes down, the block port ups and forwards the frame.