Motorola MC68824 User Manual page 152

and the destination address is the individual address of the target station. This frame goes out
from all bridges on the originating LAN. (The bridge TBC will receive the frame, the host must
either add the information to the RC field and transmit the frame onto the next segment or discard
it). Each bridge scans the RD field for the number of the target segment it attaches to and, if it is
there, the bridge will not forward the frame because it has already been on that segment. If the
bridge's next segment number is not there, the bridge adds its segment and bridge numbers to
the frame's RI field, increments the RI length by two, and forwards the frame. A number of
differently routed frames may arrive at the target station. The routing designator is copied for
each bridge as the frame travels through in the network. (The host must add this information
before transmitting the frame onto the connecting segment). The first routing designator is the
RD of the first bridge to copy the broadcast frame. The target station responds to each differently
routed frame by sending it back to the transmitting station. (The target host must change the RC
field and put the frame into the transmit queue). Each follows the route of its routing designator
field in the opposite direction. In these frames that are sent back, the RII bit is set to one, the RI
field is as it arrived, the broadcast field is set to OXX, the D bit equals one, the SA is the individual
address of the target, and the DA is the individual address of the station that sent the original
broadcast frame. The sending station receives as many responses as there are routes. The host
at the sending station chooses the route according to its specific criteria, then saves this route
and uses it for all subsequent communications with that DA. The target learns of this selected
route when it receives its first non-broadcast frame.
The host is responsible for changing the routing control field if necessary. The TBC does not
interpret the routing control field. The TBC transmits a source routing frame exactly as it appears
in its transmit queue (i.e., the host must prepare the frame according to the source routing
protocol).
More detail can be found on bridging in: IEEE 802.5, IEEE 802.1, and MAP.
MC68824 USER'S MANUAL
MOTOROLA
("_"7