v      Bridges operate in both the physical and data link layer of OSI model. Like repeaters, bridges also can be used to connect two network segments and can connect dissimilar physical media. However, bridges can also limit the traffic on each segment and eliminate bottlenecks.

v      Originally designed to interconnect Ethernet segments together.

v      Most bridges today support filtering and forwarding, as well as Spanning Tree Algorithm.
The IEEE 802.1D specification is the standard for bridges.

v      During initialization, the bridge learns about the network and the routes. Packets are passed onto other network segments based on the MAC layer. Each time the bridge is presented with a frame, the source address is stored. The bridge builds up a table which identifies the segment to which the device is located on. This internal table is then used to determine which segment incoming frames should be forwarded to. The size of this table is important; especially if the network has a large number of workstations/servers.

v      Transparent bridges (also known as spanning tree, IEEE 802.1 D) make all routing decisions. The bridge is said to be transparent (invisible) to the workstations. The bridge will automatically initialize itself and configure it sown routing information after it has been enabled.

v      Bridges are ideally used in environments where there a number of well defined workgroups, each operating more or less independent of each other, with occasional access to servers outside of their localized workgroup or network segment. Bridges do not offer performance improvements when used in diverse or scattered workgroups, where the majority of access occurs outside of the local segment.

v      The diagram below shows two separate network segments connected via a bridge. Note that each segment must have a unique network address number in order for the bridge to be able to forward packets from one segment to the other.

v      Ideally, if workstations on network segment A needed access to a server, the best place to locate that server is on the same segment as the workstations, as this minimises traffic on the other segment, and avoids the delay incurred by the bridge.

v      Operate at the MAC layer (layer 2 of the OSI model)

v      Can reduce traffic on other segments

v      Broadcasts are forwarded to every segment

v      Most allow remote access and configuration

v      Often SNMP enabled

v      Loops can be used (redundant paths) if using spanning tree algorithm

v      Small delays introduced

v      Fault tolerant by isolating fault segments and reconfiguring paths inthe event of failure

v      Not efficient with complex networks

v      Redundant paths to other networks are not used (would be useful if the major path being used was overloaded)

v      Shortest path is not always chosen by spanning tree algorithm

v      A bridge’s primary function is to filter traffic between network segments.

v      As a packet is received from a network segment, the bridge looks at the physical destination address of the packet before forwarding the packet on to other segments.

v      If the packet’s destination is on another network segment, the bridge retransmits the packet. However, if the destination is on the same network segment, on which the packet was received, the bridge assumes the packet has already reached its destination and the packet is discarded. As a result, network traffic is greatly reduced.

v      Bridges work at the data link layer of the OSI model. At this layer the hardware address, both source and destination, is added to the packet. Because bridges function at this layer, they have access to this address information.

v      Each computer in the network is given a unique address. Bridges analyze these address to determine whether on not to forward a packet.

v      In above figure, the packet generated by computer C is intended for computer K. The bridge allows the packet to cross and relay it to the entire lower segment where it is received by computer K. IF a packet is destined on a same segment (for example from computer A to computer F) the bridge will block the packet from crossing into lower segment to reduce the traffic.

v      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.

v      There are four types of bridges:

v      Simple

v      Learning

v      Multiport and

v      Translation

v      Simple bridges are the least expensive types of bridge.

v      A simple bridge links two segments and contains a table that lists the address of all the stations included in each segment. Before a simple bridge can be used, operator must program the addresses of every station.

v      Whenever a new station is added or removed, the table must be updated.

v      Installation and maintenance of simple bridges are time consuming and in long run more trouble than the cost savings are worth.

v      A bridge builds its table of station addresses on its own, as it performs its bridging function. When the learning bridge is first installed, its table is empty.

v      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 recognise the destination address, it relays the packet to all of the station on both segments.

v      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.

v      Using the same algorithm, the learning bridge is also self-updating.

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

v      Translation bridges are available that can connect different types of networks.

v      For example a translation bridge can be used to connect an Ethernet network to a token ring network.

v      Easy to extend network distances

v      Can filter traffic to ease congestion

v      Can connect network with different media

v      Translation bridges can connect different network architectures

v      Slower than repeaters

v      More expensive than repeaters

v      Can not handle multiple paths