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
Addresses
The method of
learning
station addresses, specified in the IEEE 802.1d
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:
1
The bridge receives every packet transmitted by its attached LANs.
2
For 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.
3
For 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
).
4
The 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.
abtthgde.book Page 7 Tuesday, June 23, 1998 10:29 AM
