Transparent Bridging for Ethernet LANs

Forwarder and

The Forwarder cleans up all entries in the Forwarding Table when there are spanning

STPE

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.

Learn Only Period

 

 

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

 

 

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.

 

 

 

 

52

Bridging

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EXP Computer S200 manual Learn Only Period, Aging