Bridges

Bridges operate in both the physical and the data link layers of the OSI model.Bridges divide a large network into smaller segments.They can also relay frames between two originally separate segments of one type. Unlike repeaters.However,bridges contain logic that allows them to keep the traffic for each segment separate.Bridges are repeaters that are smart enough to relay a frame only to the side of the segment containing the intended recipient.In this way,they filter traffic.

a fact that makes them useful for controlling congestion and isolating problem links.Bridges can also provide security through this partitioning of traffic.

Bridges do not modify the structure or contents of a packet in any way and can therefore be used only between segments that use the same protocol.

A bridge operates at the data link layer, giving it access to the physical addresses of all stations connected to it. When a frame enters a bridge,the bridge not only regenerates the signal but checks the address of the destination and forwards the new copy only to the segment to which the address belongs.As a bridge encounters a packet it reads the address contained in the frame, and compares that address with a table of all the stations on both segments.When it finds a match, It discovers to which segment the station belongs and relays the packet only to that segment.

For example ,the figure as above in point (a) shows two segments joined by a bridge. A packet from station A addressed to station D arrives at the bridge. Station A is on the same segment as station D, therefore the packet is blocked from crossing into the lower segment.Instead the packet is relayed to the entire upper segment and received by station D.

The same,through the figure in point (b),a packet generated by station A is intended for station G. The bridge allows the packet to cross and relays it to the entire lower segment. Where it is received by station G.

Types of Bridges

To select between segments,a bridge must have a look-up table that contains the physical addresses of every station connected to it. The table indicates to which segment each station belongs.How this table is generated and how many segments are connected by a single bridge determine the type and cost of the bridge. There are three types of bridges : Simple, learning, and multiport.

1.Simple Bridges

Simple bridges are the most primitive and least expensive type of bridge. A simple bridge links two segments and contains a table that lists the addresses of all the stations included in each to them. What makes it primitive is that these addresses must be entered manually.Before a simple bridge can be used,an operator must sit down and program the addresses of every station.Whenever a new station is added.The table must be modified.If a station is removed,the newly invalid address must be deleted. The logic included in a simple bridge.Therefore, is of the pass/no pass variety,a configuration that makes a simple bridge straightforward and inexpensive to manufacture.Installation and maintenance of simple bridges are time-consuming and potentially more trouble than the cost savings are worth.

2.Learning Bridges

A learning bridge builds its table of station addresses on its own,as it performs its bridge functions,When the learning bridge is first installed.Its table is empty.As it encounters each packet.it looks at both the destination and the source addresses.It checks the destination to decide where to send the packet.If it does not yet recognize the destination address.It relays the packet to all of the stations on both segments.It uses the source address to build its table.As it reads the source address,it notes which side the packet came from and associates that address with the segment to which it belongs. For example : if the bridge in figure above is a learning bridge then when station A sends its packet to station G.The bridge learns that packets coming from A are coming from the upper segment and that station A must be located in the upper segment.Now,wherever the bridge encounters packets addressed to A.It knows to relay them only to the upper segment.

With the first packet transmitted by each station.The bridge learns the segment associated with that station.Eventually it has a complete table of station addresses and their respective segment stored in its memory.

By continuing this process even after the table is complete,a learning bridge is also self-updating.Suppose the person at station A trades offices with the person at station G and they both take their computers(Including their NICs) with them.All of a sudden,the stored segment locations for both stations are wrong. But because the bridge is constantly checking the source address of received packets.It notices that packets form station A are now coming from the lower segment and that packets from station G are coming from the upper segment and updates its table accordingly.

The logic required to achieve this kind of automation makes a learning bridge more expensive than a simple bridge. For most applications,however,the convenience is worth the cost.

3.Multiport Bridges

A multiport bridge can be either simple or learning, and it used to interconnect more than two same-type segments.