3Com 7600 manual Transparent Bridges

Transparent Bridges 11-7

Forwarding Database The effective bandwidth of the bridged LAN can be increased by means of the Forwarding Database (FDB), a memory cache in the bridge which contains a table of station addresses and associated bridge port numbers. The FDB provides the bridge port number of a received packet’s destination station, allowing the bridge to transmit the packet directly to the correct\ LAN. The FDB is populated in three ways: by learning, by manual operator entry and by predefined data.

Learning Station The method of learning station addresses, specified in the IEEE 802.1d Addresses standard, is based on the presence of the transmitting station’s (source)

address in the transmitted packet. This makes it possible for the bridge to associate the source address with the bridge port on which the packet was received; the bridge now knows to which port the source station is connected. This information is stored in the FDB and remains there as long as the transmitting station is active. Because the learned addresses are not permanently written in the FDB, they are referred to as dynamic addresses.

Learning while forwarding

The learning process is executed by the bridge at the same time that frame forwarding is occurring. The strategy goes as follows:

1The bridge receives every packet transmitted by its attached LANs.

2For each packet received, the bridge stores the address in the packet’s source address field in the FDB together with the bridge port upon which the packet was received.

3For each packet received, the bridge looks through its FDB for the packet’s destination address.

a If the address is not found in the FDB, the bridge forwards the packet to all LANs except the one from which it was received (flooding).

b If the address is found in the FDB, the bridge forwards the packet only to the port specified in the table. If the specified port is the one from which the packet was received, the packet is dropped (filtered).

4The bridge keeps track of each entry in the FDB and deletes it after a period of time (the aging time) in which no traffic is received with that address as the source address. This prevents overflowing of the FDB and also facilitates moving a station from one LAN to another. The bridge simply relearns its location. Aging time is one of the bridge parameters and can be specified by management.