EXP Computer S200 manual Forwarder Database and Spanning Tree

Transparent Bridging for Ethernet LANs

Forwarder Database and Spanning Tree

How They Work	There is a close relationship between the forwarding database and the spanning tree.
Together	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 can be deleted from CMEM by CTP command. If the
Forwarding Table	system administrator changes the topology (changes bridges or stations), the CMEM
Entries	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.

Bridging 53

T0008-16F Release 5.2M