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Filtering Database 5-21
Bridging
Changing Path Cost
To change the Path Cost:
1. If necessary, select the desired port by clicking the mouse to highlight the port
in the lower right quadrant of the window. The lower left quadrant of the
window will now allow you to edit parameters for the selected port.
2. Highlight the Path Cost field, and type in a new value from 1 to 65535 decimal
(default is 100 decimal).
3. Click on Set.
The new path cost will be applied to the port.
Filtering Database
The Filtering Database, which makes up the IEEE 802.1 Source Address Table, is
used to determine which frames will be forwarded or filtered between the
selected module’s bridging ports.
Transparent bridges like the 9A128-01 use the Filtering Database to determine a
packet’s route through the bridge. During initialization, the 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 checks it against 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.
The Filtering Database consists of two separate databases: the Static and the
Learned Databases.
The Static Database contains addresses that are entered by a network
administrator. You add these addresses directly to the database while the bridge is
powered up, or to the selected module’s battery-backed RAM so that they are
stored on shutdown till the next power-up.
The Learned Database consists of addresses that accumulate as part of the
bridge’s learning process as it is up and running. These do not remain in the
Source Address Table when the system is shut down. The Learned Database also
contains the addresses that are in the Static Database upon start-up of the bridge.
Entries to the Source Address Table are one of four types: Permanent, Static,
Dynamic, or Learned.