3Com PathBuilder S200 Series Owner's Manual page 56

Transparent Bridging for Ethernet LANs
Forwarder and
STPE
Learn Only Period
Aging
52
The Forwarder cleans up all entries in the Forwarding Table when there are spanning
tree topology changes taking place. These actions are considered services the
Forwarder provides to the Spanning Tree Protocol Entity (STPE). The Forwarder
provides these services through MACRO routines so that they are accessible to the
entire system.
Note
The Forwarder is not required to pass STPE traffic to the handlers. The STPE is
considered to be an independent entity and similar to all the forwarders in the
system. It has its own interface with the network handlers and it uses this
interface passing PDUs to the network via the handlers. For example, it forwards
Spanning tree PDUs directly to the handler and does not go through the
Forwarder.
The Learn Only Period is a timer you can set from the CTP. This timer is started
whenever the node boots. Until this timer expires, the bridges learn only LAN station
addresses and place them into the forwarding database. The bridges do not forward
any frames during this interval. When the timer expires, the bridges forward frames
in the usual way. The default setting for this parameter is 10 seconds.
This timer is not to be confused with the Forward Delay timer of the spanning tree
protocol entity (STPE). The STPE timer is used to control how long a bridge link
withholds a link from going into the forward state once it is determined that the link
should be part of the spanning tree. This timer is set to avoid bridge topology loops
from forming. The Learn Only Period timer prevents the bridge from sending
broadcasts (as part of the learning process) when the bridge first comes up, and has a
sparse forwarding database.
Aging is an important process associated with the learned database entries. When a
new entry is learned and placed in the forwarding database, a timer is set that
indicates the station with the MAC address is still active. If the timer expires for an
entry, the entry is removed from the database. The aging time for learned entries in
the forwarding table is configurable by the system administrator. This parameter is
located in the Bridge Parameters menu. The default setting for this parameter is 3600
seconds (1 hour).
The Forwarder starts the Aging Timer for the learned addresses in the Forwarding
Table.
The purpose for aging database entries is to allow changes in the network
configuration to be automatically accounted for in the forwarding table. If a station is
moved from one LAN to another, the station becomes reachable when the entry ages
out and is replaced with a new entry that indicates new forwarding information.
Aging does not apply to the permanent entries in the forwarding table. These entries
are maintained by the system administrator and kept in CMEM. They are never aged
out of the forwarding database and they are never corrected. If a frame arrives on an
unexpected link with a source address in the permanent part of the forwarding table,
the table is not changed.
Bridging