29-12-2012, 01:27 PM
Bridges
Bridges.pptx (Size: 169.71 KB / Downloads: 27)
INTRODUCTION
A bridge operates in both the physical and the data link layer.
As a physical layer device, it regenerates the signal it receives.
As a data link layer device, the bridge can check the physical addresses contained in the frame.
A bridge has the filtering capability.
It can check the destination address of a frame and decide if the frame should be forwarded or dropped.
If the frame is to be forwarded, the decision must specify the port.
A bridge has a table that maps addresses to ports.
Transparent Bridge
A transparent bridge is a bridge in which the stations are completely unaware of the bridges existence.
If the bridge is added or deleted from the system, reconfiguration of the stations is unnecessary.
A system or network with transparent bridges must meet three criteria.
Criteria for Network with Transparent Bridge
The following are the criterias :-
Frames must be forwarded from one station to another. A transparent bridge must correctly forward the frames, as discussed in the previous example.
The forwarding table is automatically made by learning frame movements in the network.
Loops in the system must be prevented.
Learning by Transparent Bridge
The earliest bridges have forwarding tables that were static.
The system administrator would manually enter each table entry during bridge setup.
Although the process was simple, it was not practical. If a station was added or deleted, the table had to be modified manually.
The same is true, if a stations MAC address is changed.
Loop Problem in Transparent Bridges
Transparent bridges work fine as long as there are no redundant bridges in the system.
However the system administrator likes to have redundant bridges to make the system more reliable.
Redundancy can create the loop in the system, which is very undesirable.