3-3
Operation
Switching
The 9T425-16, 9T427-16, and 9T428-16 operate as a Source Route Translational
(SR-TB) switch. They provide translation to transparent frames. For information
on conÞguring the switching function, see the addendum to the SmartSwitch 9000
Local Management UserÕs Guide, containing information speciÞc to this model.
All SR (Source Route) and TP (Transparent) frames are translated to canonical
frames (even between ports on the front panel). The RIFs (Routing Information
Field) of SR frames are cached on each port.
In Traditional Bridging mode, all canonical frames are forwarded based on a
destination address. RIF information is not used in the decision to forward
frames.
The Token Ring ports on the 9T425-16, 9T427-16, and 9T428-16 are conÞgurable
for three modes of operation:
TP (Transparent) - ports receive Source Route and Transparent frames but
transmit only packets that are Transparent.
SR (Source Routing) - ports receive and transmit only packets that are Source
Route.
SRT (Source Route and Transparent) - ports receive and transmit both Source
Route and Transparent packets.
Transparent Switching
Transparent Switching is accomplished by building a Source Address Table (SAT)
from source MAC/physical addresses and using the SAT to make forwarding
decisions.
The 9T425-16, 9T427-16, and 9T428-16 prevent unnecessary network trafÞc from
passing through the module by implementing the IEEE 802.1d Þltering process.
The IEEE 802.1d Þltering process, begins with the creation of a list of local node
addresses in a table (the SAT). When the 9T425-16, 9T427-16, and 9T428-16 Þrst go
on-line, they initially forward all packets across the bridge. After receiving a
packet on the bridge port, the 9T425-16, 9T427-16, and 9T428-16 learn 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 9T425-16, 9T427-16, and
9T428-16 does not forward across the Token Ring link.