Computer Network Learning-bridge (2)

Bridge: A technology used to forward frames. It works at the data link layer and forwards and filters received Frames Based on the target address of the MAC frame,

When the bridge receives a frame, it first checks the target MAC address of the frame and then goes to the forwarding table to check whether the MAC address exists. If yes, check whether the destination address of the frame belongs to the same network segment as the source address. If yes, the packet is discarded. Otherwise, the packet is forwarded from the corresponding port. If the MAC layer address is not found, this frame is forwarded from all other interfaces.

Benefits of bridges:

(1) filter traffic and increase throughput

(2) Expand the physical scope

(3) interconnection between different physical layers, different MAC Sub-layers, and Ethernet at different rates

(4) improved reliability

Disadvantages of the bridge:

(1) The bridge will first store the received frames, then find the forwarding table, and perform CSMA/CD before forwarding.AlgorithmAdded latency;

(2) bridges are only applicable to Ethernet networks with few users and low traffic. Otherwise, a broadcast storm may occur;

(3) the MAC Sub-layer does not have traffic control. Therefore, when the traffic is large, the cache in the bridge may be insufficient and the cache overflows, resulting in Frame loss;

Currently, transparent bridges are the most used.

Transparency means that the sent frame does not know which bridges are going through during transmission.

A transparent bridge is an out-of-the-box plug-and-play device that can work as long as it is connected to Ethernet without manual configuration. The transparent bridge does not require manual configuration. It uses a self-learning method. The principle is as follows: frames sent from a station a are transmitted to the bridge through interface X, then we know that through interface x reverse transmission, we can certainly reach Site A. At this time, we will write down the source address and interface number of this frame and add it as a project to the forwarding table.

The bridge works in the storage and forwarding mode, that is, the whole frame is first stored and then processed, while the hub or forwarder processes the frames one by one. In addition, the bridge will also discard the frames with errors in CRC verification and invalid frames that are too long or too short.

Frame Forwarding process: Check whether there are items in the forwarding table that match the target address of the frame. If so, the items are forwarded from the corresponding interface. If not, the items are forwarded from all other interfaces. It should be noted that if the forwarded interface is the interface from which the frame enters the bridge, the frame will be discarded (because the source address and the destination address are in the same network segment at this time and do not need to be forwarded by the bridge ).