A Bridge is another device used to connect LAN cable segments together. Bridge operates at the data link layer of OSI model.
A Bridge provides packet filtering at data link layer, meaning that it only passes the packets that are destinated for the other side of the network.
a Bridge has two or more network interface (own MAC address)connected to different cable segments and operating promiscuous mode.
A Bridge examines each packet as it enteres through one of the ports. It first looks at the MAC address of the sender and creates a mapping between the port and the sender's MAC address. It then looks at the address of the ecipient , comparing the MAC address to the list of all learned MAC address. If the address is in list. the Bridge look up the port number and forwards the packet to the port where it thinks the recipient is connected. If the rrecepient's MAC address is not in the list , the Bridge blocks the data from passing.
Bridge can connect different network architectures like Ethernet and Token Ring.
Types of Bridges.
Three types of Bridges are used in network.
1> Transperent Bridge.
2> Translational Bridge.
3> Source-route Bridge.
Transperent Bridge :
Transperent Bridges is invisible to the other evices on the network. Transperent Bridge only perform the function of blocking or forwarding data based on MAC address. MAC address may also be referred as hardware address or physical address. These addresses are used to built tables and make decision regarding whether a frame should be forward and where it should be forwarded.
Translational Bridge :
Translational Bridges are usefull to connect segments running at different speeds or using different protocols such as token Ring and Ethernet networks. Depending onthe direction of travel, a Translational Bridge can add or remove information and fi elds from frame as needed.
Source-route Bridge :
Source-route Bridges were designed by IBM for use on Token ring networks. the sr Bridge derives the entire route of the frame embedded within the frame. This allows the Bridge to make specific decision about how the frame should be forwarded through the network.
Other than above, there are also Local Bridge, Remote Bridge.
Local Bridge :
A Local Bridge provides paket filtering and repeating services fot the network segments of the same type. this is also called MAC-layer Bridge. This is the simplest type of Bridge because it has no need for packet filtering or buffering. This Bridge simply propagates the incoming packets to the appropriate ports or discard them.
Remote Bridge :
Remote Bridge connects network segments at different locatios, using WAN link such as MODEM or Leased line. The difference in speed between Local and wide area links, a remote Bridge uses internal buffer to store the data received from LAN while it is waiting for transmission to the remote site.
Advantages of Bridges :
1> Bridges are simple and significant.
2> They prevent unnecessary traffic from crossing onto other network segments.
3> Bridge can reduce the amount of network traffic on segments.
4> It also make it possible to isolate a busy network from not-so-busy network.
5> They can connect different network architectures like Ethernet & Token ring.
6> Bridge have ability to look at the physical destination address of the frame and send the frame at the specific port.
7> Bridge can filter the traffic, it increases throughput on a network.
Disadvantages of Bridges :
1> Bridges are slower than Hubs & Repeaters, because they examine each frame's source and deatination addresses.
2> Bridges have no protection against broadcast storms. that is, when a computer needs to send information to all other computers on the networks, it sends a Broadcast frame. Bridges do not have the ability to stop Bridgeoadcast packets from being forwarded. When a Bridge receives a frame with a Broadcast destination address. the Bridge will simply forward the frame out every one of its ports.
3> A Bridge can not make decissions about routes through the network.