Introduction
Orbitor 3000 Reference Manual— 1-13
Forwarding Unknown Destination Addresses
When a frame is received from a LAN segment with an unknown destination address (an address that does not
yet exist in the filter table), the bridge will forward the frame to the other segment, logging the address, and
marking the location as “unknown.”
Unknown Location Update
When the receiving station transmits a frame in the opposite direction, the bridge will now see the previously
unknown destination address in the source address field. It will now process this source address as it did during
the initial learning stage, adding the location to the address entry.
In this fashion (looking at source addresses of non-local packets), the bridge learns about non-local stations and
their associated arrival ports. The bridge then updates the location of each address in its table. In the future the
bridge will look up these stored non-local addresses to determine the bridge port on which to forward a packet
destined for a known non-local station.
In summary, the Orbitor Ethernet bridge/router will “learn” the location of a station by examining the source
Ethernet address, and will “filter” frames based on destination address. A frame received from one segment that
is of “unknown” location will be forwarded to the other segment. A frame that is received with a source
address equal to a known address, but previously marked as an unknown location, will be updated in the filter
table to add the location.
Aging Timer
During the bridging process, the filter table is built giving the location (bridge port or LAN segment) of known Ethernet
addresses. The table would become quite large, eventually reducing performance, if stations were added, removed, or
moved without the old information being purged periodically. Performance is affected since the larger the table, the more
time it will take to process an incoming frame.
This purging process, called “aging,” is an integral part of the learning function. It limits the size of the filter table and
ensure that performance is not reduced unnecessarily.
Aging assumes that many of the addresses may not be active all of the time, and could be purged after a specified interval
to keep the size of the filter table small. In general terms, the smaller the table, the higher the performance.
Address Purging
To achieve this routine housekeeping, the filter table contains the LAN addresses, along with their LAN port
identifier, and a timer flag. Each time a particular address is looked up or added to the table, a timer flag is set
for the “fresh” entry. When a time interval, defined by the Bridge/Router Manager expires, the address table is
scanned and any “stale” entries that have not been used since the timer expired are removed. This timer is called
the “aging timer” and may be controlled through the bridge options.
Purging the address does not prevent the station from using the bridging facilities, since the location of the station
may be re-learned. However, there must be a balance, since a small aging timer value will mean that the bridge
must learn many addresses often. This also has an effect on performance.
Aging Exception
“Permanent” address entries are an exception to the aging rule. A permanent address is one that is not subject to
the aging timer and will remain in the filter table for an indefinite period of time.
