Thursday, 28 February 2013

What is ethernet Bridge?

Ques: What is Ethernet Bridge ?

Ans: Before understanding the term "Bridge", we should have some basic knowledge of how Hub works in computer networking. In the previous article we have discussed about NIC and HUB and so far we have known hub's function and its some disadvantage. In this article we will see how bridge works and how it overcomes some disadvantage of HUB.

Bridge is hardware device as HUB but basically used to connect two different segments of network. In simple word, Bridge breaks up collision domain. HUB and connected device make a big and large collision domain where every active port of bridge creates single collision domain. Thus it reduces the wasting of bandwidth and reduces chances of collision of data. Look the below figure.



hub and bridge collision domain 


In the above figure, upper portion dictates 1 collision domain because hub is connected with another hubs and other devices (in this case computers) and the bottom portion dictates that bridge is connected with three hubs and hubs are connected with computers. Since, Bridge breakup collision domain, thus here is 3 collision domain 1 collision domain/active port.

Bridge receives frame and learns MAC address and store MAC address in mac-address-table and then forward the frame to appropriate port. HUB is not capable of learning any address. So we can say that Bridge is Layer 2 device. Look figure below:



 bridge mac-address-table

To switch the frame from one port to another port three switching mechanism are available in layer 2.


  • Store and forward mechanism

  • Fast Forwarding (Fragment free)

  • Cut-through

Bridge use only Store and forward mechanism for forwarding the frame.To know how bridge decides to forward the frame click here.

Redundant like causes loop but Bridge provides redundant link and supports STP (Spanning Tree Protocol) to avoid bridging loop. STP will be discussed later.

Summery of Ethernet Bridge :


  • Bridge is Layer 2 (Data Link) device.
  • It connects two different segments.
  • It has main three function
    • Learning MAC address
      • learning mac address is performed in software
      • Inactive mac-address in mac-address-table is removed after 300 secs.
    • Forwarding Frame and
      • Store and forward mechanism is used to forward frame.
    • Avoiding loop
      • Only STP is used.
  • Every active port is one collision domain.
  • Bridge and connected device create one broadcast domain.
  • It supports upto 10 Mbps.
  • it is half duplex device.
  • CSMA/CD is enabled.
  • Maximum port is 8

See also :


Bridge and its function.


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