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NB-30 Bridging
3-24 Filtering Database
Changing Port Priority
To change the part of the Port Priority used in priority comparisons:
1. Highlight the port Priority field for the desired port, and enter the new priority
identifier. Only valid hexadecimal numbers (0 to FF) are allowed in this field.
The default is 80 hexadecimal.
2. Click on Set. The new port priority will be saved.
Changing Path Cost
To change the Path Cost:
1. Highlight the Path Cost field for the desired port, and type in a new value from
1 to 65535 decimal (default is 100 decimal).
2. Click on Set.
The new path cost will be applied to the port.
Filtering Database
The Filtering Database, which makes up the IEEE 802.1d Source Address Table, is
used to determine which frames will be forwarded across the bridge from one
network segment to another.
Transparent bridges like the NB-30 use the Filtering Database to determine a
packetÕs 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 then 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 Þlters
(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 NB-30 Filtering Database consists of two separate databases: the Static and the
LearnedDatabases.
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 deviceÕs battery-backed RAM so that they are stored on
shutdown until the next power-up.
The Learned Database consists of addresses that accumulate as part of the
bridgeÕs 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.