Specifications

Introduction

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, each entry in the filter table contains the LAN addresses, the 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, since a small aging timer value will mean that the bridge must re-learn addresses more

often, there must be a balance between table size and aging time to achieve optimal 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.

A table is reserved for permanent address entries, separate from the table that is used for those non-permanent

entries that are subject to aging. These tables may be displayed and modified with the bridge/router options

discussed in this manual. Access is made locally from each Bridge/Router Console or one bridge/router can be made

Master, able to control all functions of a partner Ethernet bridge/router.

Filled Address Table

Sometimes filter address table may become full. (The filter table can hold 2048 address entries.) If this occurs, an automatic

procedure is followed.

This procedure defines that an address that is not in the table will not be added and will be treated as any other unknown

address. The frame will be passed to the other segment. An alarm will also be generated with the message “Station Address

Table Full,” and from this point, another alarm will be generated only if in the meantime the table empties by 1/3 and then

fills up again.