Cabletron Systems TSX-1620 User Manual

The Filtering Database Window

3-15

Using the TSX-1620 Bridge View

Mtu stands for “maximum transfer unit”; it is the largest frame size that can be 
processed by the TSX-1620. A port discards any received frames that are larger 
than the Mtu; this field lists how many such frames were discarded. 

The information in the Bridge Statistics window is a snapshot of the data. When 
you open the Bridge Statistics window, the application polls the devices for 
information. Devices are not polled again until you click mouse button 1 on the 
Update

 button, or close, then re-open the Statistics window.

In Transparent bridging, each bridge port uses the device’s Filtering Database to 
determine a packet’s route through the bridge. The Filtering Database is created 
from permanent entries made via management, and from entries learned as the 
bridge collects and stores the source address and port association from each 
packet it receives.

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 isn’t 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 has two types of entries: Forwarding and Static. The 
Forwarding view of the Filtering Database contain addresses that the bridge 
learns from network traffic (also known as dynamic entries) as well as all the 
static entries. Learned entries are subject to the bridge’s Dynamic Ageing Timer; 
entries that aren’t accessed within the time specified by the ageing timer are 
purged. Static entries may be subject to the ageing timer, depending on how the 
entries were added. Static entries enter the Filtering Database in two ways: either 
automatically, when permanent database entries are copied to the Filtering 
Database, or manually when you move a Forwarding entry to the Static Table.