3Com LANPLEX 2500, Operation Manual

Page: 38 / 139

How a Bridge Ages Addresses 5-3
How a Bridge
Ages Addresses
A source address remains in the address table as long as the station to which
it relates regularly transmits through the bridge. If the station does not
regularly transmit, the source address is “aged out” of the bridge’s table.
Address aging is primarily implemented to ensure that if a station moves to
a different segment on the network, its address will be forgotten at the old
location and packets will no longer be forwarded to that location. Address
aging is also necessary because a bridge can learn only a finite number of
addresses. The LANplex 2500 system, when configured as an IEEE 802.1d
bridge, can learn up to 8K addresses in its address table.
Address aging, although typically an efficient means of maintaining a
current address table, can create problems when regularly used stations on
the network do not transmit periodically. For instance, printers only transmit
when they are powered on, yet printing is a function performed frequently
on a network. In this case, the printer’s address is aged out of the address
table and the bridge no longer has the information it needs to send packets
directly to that station.
To handle this situation, the LANplex system allows you to statically configure
the addresses of these stations. Because a statically configured address is not
aged out of memory, it must be manually flushed when the station is
removed from the network. Static configuration of Ethernet addresses and
flushing static Ethernet addresses are described in the
LANplex® 2500
Administration Console User Guide .