Bridges

• have all the abilities of a repeater
• Bridges can
• take an overloaded network and split it into two networks, therefore they can divide the network to isolate traffic or problems and reduce the traffic on both segments
• expand the distance of a segment
• link UNLIKE PHYSICAL MEDIA such as twisted-pair (10Base T) and coaxial Ethernet (10Base2)
• VERY IMPORTANT: they can link UNLIKE ACCESS CONTROL METHODS, on different segments such as Ethernet and Token Ring and forward packets between them. Exam Cram says this is a Translation Bridge that can do this - not all bridges - but my observation is questions don't necessarily mention the distinction.

• Bridges work at the Data Link Layer of the OSI model => they don't distinguish one protocol from the next and simply pass protocols along the network. (use a bridge to pass NetBEUI, a non-routable protocol, along the network)
• Bridges actually work at the MEDIA ACCESS CONTROL (MAC) sublayer. In fact they are sometimes called Media Access Control layer bridges. Here's how they deal with traffic:
• They listen to all traffic. 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.
• they check the source and destination address  of each PACKET
• They build a routing table based on the SOURCE ADDRESSES. Soon they know which computers are on which segment
• Bridges are intelligent enough to do some routing:
• if the destination address is on the routing table and is on the SAME SEGMENT, the packet isn't forwarded. Therefore, the bridge can SEGMENT network traffic
• If the destination address is the routing table, and on a remote segment, the bridge forwards the packet to the correct segment
• if the destination address ISN'T on the routing table, the bridge forwards the packet to ALL segments.
• BRIDGES SIMPLY PASS ON BROADCAST MESSAGES, SO they too contribute to broadcast storms and don't help to reduce broadcast traffic
• Remote Bridges
• two segments are joined by a bridge on each side, each connected to a synchronous modem and a telephone line
• there is a possibility that data might get into a continuous loop between LANs
• The SPANNING TREE ALGORITHM (STA)
• senses the existence of more than one route
• determines which is the most efficient and
• configures the bridge to use that route
• this route can be altered if it becomes unusable.
• 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 its own routing information after it has been enabled.
• Comparison of Bridges and Repeaters
• Bridges
• regenerate data at the packet level
• accommodate more nodes than repeaters
• provide better network performance than repeaters because they segment the network
• Implementing a Bridge
• it can be an external, stand-alone piece of equipment
• or be installed on a server
• Summary from MOC:
• Bridges have all the features of a repeater
• They connect two segments and regenerate the signal at the packet level
• They function at the Data Link layer of the OSI model
• Bridges are not suited to WANs slower than 56k
• They cannot take advantage of multiple paths simultaneously
• They pass all broadcasts, possibly creating broadcast storms
• Bridges read the source and destination of each packet
• they PASS packets with unknown destinations
• Use Bridges to:
• Connect two segments to expand the length or number of nodes on the network
• reduce traffic by segmenting the network
• Connect
• unlike MEDIA ( e.g. 10BaseT and 10Base2)
• unlike ACCESS CONTROL METHODS (Ethernet and Token Ring)

The advantages of bridges are
• increase the number of attached workstations and network segments
• since bridges buffer frames, it is possible to interconnect different segments which use different MAC protocols
• since bridges work at the MAC layer, they are transparent to higher level protocols
• by subdividing the LAN into smaller segments, overall reliability is increased and the network becomes easier to maintain
• used for non routable protocols like NetBEUI which must be bridged
• help localize network traffic by only forwarding data onto other segments as required (unlike repeaters)
The disadvantages of bridges are
• the buffering of frames introduces network delays
• bridges may overload during periods of high traffic
• bridges which combine different MAC protocols require the frames to be modified before transmission onto the new segment. This causes delays
• in complex networks, data is not sent over redundant paths, and the shortest path is not always taken
• bridges pass on broadcasts, giving rise to broadcast storms on the network

Sample Question:
You want to connect an Ethernet network in one part of an office building to a Token-ring network down the hall. Both networks use NWLink IPX but must eliminate the IPX addressing and use only NetBEUI on both segments when they are joined. Which connectivity device do you choose which will allow the two networks to communicate, but at the same time reduce network levels. device should you use?

1. repeater
2. bridge
3. router
4. gateway

B - they are testing here to see if you know what a translation bridge can do.
Some bridges can't connect different segments that use different media schemes, but a translation bridge can. A translation bridge will also reduce network traffic because it can analyze packets based on MAC address and if it finds them to be from the same segment as the originating they are simply discarded instead of being passed on to a non-local segment. The bridge can do this using address information stored in its bridging table.