Bridging; Transparent Bridging - Cabletron Systems SmartSwitch 9000 User Manual

Operation

Bridging

The 9T125-08 is configured for SR-TB bridging. As shown in Figure 3-2, the
module 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 model.
Source Route
and Transparent
Packets

Transparent Bridging

Transparent Bridging is provided between all three interfaces and is
accomplished by building a Source Address Table (SAT) from source
MAC/physical addresses and using the SAT to make forwarding decisions.
The 9T125-08 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 a table (the SAT). When the 9T125-08 first goes on-
line, it initially forwards all packets across the bridge. After receiving a packet on
the bridge port, the 9T125-08 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 9T125-08 does not forward across the Token
Ring link.
3-2
TR1
SR-TB
Bridge
TR2
Figure 3-2. 9T125-08 Bridging Diagram
FNB - Transparent
Packets only