Bridge is an early two-port Layer 2 network device used to connect different network segments. The two ports of the bridge have an independent switching channel, instead of sharing a backplane bus, which can isolate the collision domain. Bridges perform better than hubs, where ports share the same backplane bus. Later, bridges were replaced by switches that had more ports while also isolating collision domains.

A bridge is like a smart repeater. Repeaters take signals from one network cable, amplify them, and send them to the next cable. In comparison, the bridge is more sensitive to the information uploaded from the level. Bridging is a technology that forwards frames to isolate collisions based on MAC partitions. A bridge connects multiple segments of a network at the data link layer. In practical applications, there are mainly four types of bridges: transparent bridges, source address routing bridges, translation bridges, and source address routing-translation bridges.

The bridge forwards according to the bridge table. The bridge table consists of two parts: MAC address and interface. When the bridge is connected to a physical network segment, it will monitor all Ethernet frames on the physical network segment. Once an Ethernet frame sent by a node on an interface is detected, the source MAC address of the frame will be extracted, and the The correspondence between the MAC address and the interface receiving the frame is added to the bridge address table.

When using a bridge to connect two segments of LAN, the bridge should first check the destination address of the MAC frame from segment 1. If the frame is sent to a station on network segment 1, the bridge will not forward the frame to network segment 2, but filter it out; if the frame is sent to a station on network segment 2, the bridge will filter it out. Then it is forwarded to network segment 2, which shows that if there are a pair of users on both LAN1 and LAN2 to communicate on this network segment at the same time, it is obviously achievable. Because the bridge plays an isolation role. It can be seen that the bridge can increase the network bandwidth under certain conditions.

Advantages and disadvantages of bridge technology:


1. Filter traffic. The network bridge can use the information volume between the workstations on a network segment of the local area network to be limited within the scope of this network segment, and will not slip to other network segments through the network bridge.

2. Expand the physical range and increase the maximum number of workstations on the entire local area network.

3. Different physical layers can be used to interconnect different local area networks.

4. Improve reliability. If the larger local area network is divided into several smaller local area networks, and the amount of information inside each small local area network is significantly higher than that between the networks, then the performance of the entire interconnected network becomes better.


1. Because the bridge needs to store and look up the station table for the received frame first, and then forward it, which increases the delay.

2. There is no flow control function in the MAC sublayer. When the network is under heavy load, it may overflow due to insufficient storage space in the bridge buffer, resulting in frame loss.

3. When network segments with different MAC sublayers are bridged together, before forwarding a frame, the bridge must modify the content of some fields of the frame to suit the requirements of another MAC sublayer. Increase the delay.

4. The network bridge is only suitable for the local area network with not too many users (not more than a few hundred) and the amount of information is not too large, otherwise a large broadcast storm will sometimes occur.

One might think that bridging from one 802 LAN to another 802 LAN is very simple, but it's not. Of the nine combinations of 802.x to 802.y, each has its own special problems to solve. Before discussing these specific issues, let's take a look at the general issues that these bridges face.

Various local area networks use different frame formats. This incompatibility is not caused by technical reasons, but simply because of the companies that support the three standards (Xerox, GM, and IBM), none of which are willing to change the standard they support. The result: copying frames between different LANs requires rearranging the format, which takes CPU time and recalculates checksums.

The second problem is that interconnected local area networks do not necessarily operate at the same data transfer rate. When a fast LAN sends a long series of consecutive frames to a slow LAN, the bridge processes the frames slower than the frames come in. Bridges must use buffers to store frames that are too late to process, while also beware of running out of memory. Even 10Mb/s 802.4 to 10Mb/s 802.3 bridges have this problem to some extent.

Possibly the most serious of all the problems is that the three 802LANs have different maximum frame lengths. For 802.3, the maximum frame length depends on configuration parameters, but for a standard 10M/bs system the maximum payload is 1500 bytes. The maximum frame length of 802.4 is fixed at 8191 bytes. 802.5 has no upper limit, as long as the station's transit time does not exceed the token holding time. If the token time defaults to 10ms, the maximum frame length is 5000 bytes.

The bridge does not know the information of the high-level protocols in the forwarding frames, which makes it possible to deal with IP, IPX and other protocols in the same way at the same time, and it also provides a network segmentation method without routing protocols. Features.

Because routers process data at the network layer, it is easier for them to interconnect different data link layers, such as Token Ring segments and Ethernet segments. Bridges are generally harder to control than routers. Protocols like IP have complex routing protocols that make it easy for network managers to manage routing; protocols like IP also provide more information on how the network is segmented (even its addresses provide such information). Bridges, on the other hand, only work with MAC addresses and physical topology. Therefore, bridges are generally suitable for small and relatively simple networks.


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