Operation
3-2

Bridging
The 9T122-24 is configured for SRT bridging. It will pass SR (Source Routing)
frames between the rings and provides translation to transparent frames over the
FNB to Ethernet and FDDI modules. For information on configuring the bridging
function, see the addendum to the SmartSwitch 9000 Local Management User’s
Guide, containing information specific to this module.

Transparent Bridging

Transparent Bridging is accomplished by building a Source Address Table (SAT)
from source MAC/physical addresses and using the SAT to make forwarding
decisions.
The 9T122-24 prevents unnecessary network traffic from passing through the
module by implementing two separate filtering processes — IEEE 802.1d or
Cabletron’s Special Filtering Database. These processes may be used individually
or in tandem.
The first process, the IEEE 802.1D filtering process, begins with the creation of a
list of local node addresses in the SAT. When the 9T122-24 first goes on-line, it
initially forwards all packets across the bridge. After receiving a packet on the
bridge port, the 9T122-24 learns the address of the sending node from the packet
and stores that address in the SAT. In this manner, the bridge learns the address of
each node on each side of the bridge. The bridge then uses the addresses stored in
the table to compare the destination address of each subsequent packet that
travels to the bridge. If the destination address of a packet resides on the bridge
segment, the 9T122-24 does not forward across the Token Ring link.
The second filtering process, the Cabletron Systems Special Filtering Database,
provides an additional step in the filter/forward decision. Through Remote
Management, you can define up to 10 additional filtering parameters for
incoming network traffic. These parameters include (but are not limited to) the:
• Destination address
• Source address
• Type field (protocol)
• 64 bytes of the data field (using a data offset)
For example, using this process, you can ensure that the 9T122-24 always filters or
forwards packets with a specific protocol or address.