Filtering Database 5-19
Bridging
Filtering Database
When the 9H42x-xx switch is using Transparent Bridging, the Filtering Database,
which makes up the IEEE 802.1d Source Address Table, is used to determine
which frames will be forwarded or filtered between the 9H42x-xx module’s
bridging interfaces.
Transparent bridges like the 9H42x-xx use the Filtering Database to determine a
packet’s route through the bridge. During initialization, a bridge copies the
contents of its Permanent Database to the Filtering Database. Next, the bridge
learns network addresses by entering the source address and port association of
each received packet into the Filtering Database. When in the Forwarding state,
the bridge examines each received packet and compares the destination address
to the contents of the Filtering Database. If the destination address is located on
the network from which the packet was received, the bridge filters (does not
forward) the packet. If the destination address is located on a different network,
the bridge forwards the packet to the appropriate network. If the destination
address is not found in the Filtering Database, the bridge forwards the packet to
all networks. To keep Filtering Database entries current, older entries are purged
after a period of time, which is called the Dynamic Ageing Time.
Entries to the Source Address Table are one of four types: Static, Permanent,
Dynamic, or Learned.
•Static entries are addresses that you add to the Static Database (via the
Filtering Database window). These entries are not subject to the ageing timer,
and will remain in the Source Address Table until the 9H42x-xx module is shut
down.
•Permanent entries are also addresses that you add to the Static Database (via
the Filtering Database window); once classified as permanent, these are stored
in the device’s battery-backed RAM and are preserved between power-up
cycles.
•Dynamic entries are addresses that you add to the Static Database (via the
Filtering Database window). These entries are subject to the Ageing Timer, and
will be automatically deleted if they do not transmit data during one complete
timer cycle. You can set the ageing timer via the Ageing Time field in the
Filtering Database window.
•Learned entries are addresses that are added to the Learned Database through
the bridge’s learning process. Like Dynamic entries, these entries are subject to
the Ageing Timer, and will be automatically deleted if they do not transmit
data during one complete timer cycle.
Learned address entries are divided into two types: Learned and Self. Address
entries classified as Learned have transmitted frames destined for a device
attached to a network segment installed in the 9H42x-xx module; address
entries classified as Self are those that have sent a frame with a source address
of one of the 9H42x-xx module’s available bridge port interfaces.