EXP Computer PathBuilder S200 - Manual (Page 57)

Bridging 53
T0008-16F Release 5.2M
Transparent Bridging for Ethernet LANs
Forwarder Database and Spanning Tree
How They Work
Together There is a close relationship between the forwarding database and the spanning tree.
The spanning tree can be manually configured. This is a reasonable thing to do in the
case where a stable environment exists since it saves CPU processing by eliminating
aging timers and the broadcasting that is employed when the forwarding table does
not have a suitable entry. In this case, when the node is booted, bridges will form a
spanning tree (always the same one provided all equipment remains operational),
and a permanent forwarding database can be loaded from CMEM that is required for
the configuration. This also allows a quick method for the bridge to become
operational.
The learning process continues even if the forwarding table is formed initially from
permanent CMEM entries. That is, the forwarding database adds learned entries as
they occur; if there are stations active that are not in the initial database, they will be
added as they are learned. Such an expanded database can be written to the CMEM
by a CTP update command. This has the effect of converting the entire forwarding
table in running memory to permanent entries and creating a new permanent table in
CMEM equal to the running configuration.
If the spanning tree is configured for automatic configuration, then the operation of
the forwarding database is as noted previously. The permanent database offers a
means of quickly obtaining a forwarding database without the bridge having to
broadcast frames for which it does not know the destination link. However, since the
bridge topology can change in an unpredictable way (corresponding to unpredictable
network failures), it is best that the entries in the database are all aged. Aging all
entries allows the forwarding database to remain current even with topology
changes. Therefore, it is recommended that if such changes are expected, the system
administrator should not use permanent forwarding entries. However, there is no
reason that they cannot be used, and the full set of editing and saving commands still
apply when the spanning tree is in automatic operation.
Deleting
Forwarding Table
Entries Forwarding table entries can be deleted from CMEM by CTP command. If the
system administrator changes the topology (changes bridges or stations), the CMEM
record (edit, delete, add) can be updated and the table booted to get a cleaned up
version of the database in working memory. This boot does not disrupt bridge
operation other than a momentary disruption to forwarding traffic.
The entire forwarding table in CMEM can also be deleted by a single CTP
command. This lets you make substantial changes to the topology, then boot the
bridge network (with no permanent forwarding table entries) to let the bridge learn
station locations. After a suitable learning time, you can update the entries to
permanent CMEM entries using the update command. From that point on, the
permanent forwarding table will contain valid entries for the configuration.