Transparent bridge
The first 802 bridge is a transparent bridge or a spanning tree bridge. People who support this design are completely transparent. According to their point of view, when you have multiple LANs, after buying back to the IEEE Standard Bridge, just insert the connection plug into the bridge, do everything. There is no need to change hardware and software, there is no need to set the address switch, no need to load the routing table or parameters. In short, there is nothing, it is only necessary to insert a cable, and the existing LAN is not affected by the bridge. This is really incredible, they finally succeeded.
Transparent bridges work in a mixed manner, which receives each frame of all LANs transmitted. When a frame arrives, the bridge must decide to discard it or forward it. If you want to forward, you must decide which LAN sent to. This requires decisions by querying the destination address in a large-scale list of bridges. This table can list each possible destination and which output line it belongs to it. At the beginning of the bridge, all have a meter. Since the bridge does not know the location of any destination, the diffusion algorithm is used to output each arrival, the unknown frame of the destination is used to all LANs connected to this bridge (except for the LAN of the frame). . As time goes by, the bridge will understand the location of each destination. Once the destination location is known, the frame to the site is only placed on the appropriate LAN, and no longer distributes.
The algorithm used by the transparent bridge is the backward learning. The bridge works in a mixed manner, so it can see the frames transmitted on any LAN connected. You can know which machine on which LAN can be accessed on which LAN is added to the source address.
When computers and bridges are powered, power off or migrated, the topology of the network will change. In order to handle dynamic topology, each time the hash entry is added, the arrival time of the frame is indicated in this item. Whenever the destination has arrived in the frame in the table, the item will be updated at the current time. Thus, the time of the machine last frame can be known from each time from the table. There is a process periodically scanning a list of regularly, and the current time is long than the current time several minutes. So, if you take a computer from the LAN and re-connect to the LAN elsewhere, it can restart the normal operation without manual intervention. This algorithm also means that if the machine has no action in a few minutes, the frame sent to it will have to be distributed, and it will send an frame to it.
The routing process of arrival frame depends on the LAN (source LAN) and the LAN (destination LAN) in the destination, as shown below:
1. If the source LAN is the same as the destination LAN, the frame is discarded.
2. If the source LAN is different from the destination LAN, then the frame is forwarded.
3. If the destination LAN is unknown, spread.
In order to improve reliability, some people set two or more bridges between the LANs, but this configuration causes another problem because a loop is generated in the topology, which may trigger an infinite loop.
