Motorola MC68824 User Manual page 148

Bridge B's individual address mask is set to CO. The address mechanism would work as follows:
IA 1100 0000 IA 1100 0000
DA 1010 0101 TS 01xx xxx x
IA AND DA 1000 0000 IA AND TS 0100 0000
The equation individual address (lA) AND destination address (DA)
=
IA AND this station (TS) is
not true, so bridge B does not accept the frame.
Bridge C's individual address mask is set to 03. The address mechanism would work as follows:
IA 0011 0000 IA 0011 0000
DA 1010 0101 TS 0010 xxxx
IA AND DA 0010 0000 IA AND TS 0010 0000
The equation individual address (lA) AND destination address (DA)
=
IA AND this station (TS) is
true, so bridge C accepts the frame.
C.2.2 Lower Bridges
Each lower bridge is responsible for passing frames up from the segment to the backbone. In
order to be a lower bridge, the LBRM bit must be set using the SET MODE 2 command. In lower
bridge mode, the bridge will forward frames only when the frame is not addressed to a station
below the bridge (lA mask AND DA are not equal to IA mask AND TS or GA mask AND DA are
not equal to DA). This means that lower Bridge C will forward to region 1 only those frames which
do not belong in segment 1.
C.3 FLAT ADDRESSED BRIDGING OVERVIEW
Flat addressed bridges can interconnect segments where the addresses have been assigned
randomly. A Spanning Tree uses'flat addresses yet it is a hierarchical bridge. The Spanning Tree
approach is currently being investigated by the IEEE 802.1 committee.
A table of translations is required at each station in some flat addressed bridging schemes such
as IBM defined source routing. In source routing, the route taken by, or chosen for a frame as it
traverses the network is reflected in the routing information field which is imbedded in the frame
itself. Source routing is being proposed by the IEEE 802.5 committee for interconnection of 802.5
segments. The token bus controller provides the MAC functions necessary to implement a source
routing bridge or station, making it possible to interconnect 802.4 segments into 802.5 networks
or to other 802.4 segments. Stations not implementing source routing can coexist on the same
segment (network) with stations that do use source routing, but cannot send messages through
bridges using source routing.
If a station wishing to send a frame to another segment already knows the routing information
on how to get it there, the station sends the frame with the routing information imbedded in the
II
routing information field of the frame. If the station that wishes to send the frame does not know
the routing information on how to get there, the station can dynamically discover the necessary
routing information. The routing information is obtained by the station sending a "broadcast"
frame which travels over every bridge in the network, eventually reaching its destination. On their
way, broadcast frames record where they have been. If a bridge receives a broadcast frame which
, has already been on the segment it connects to (it finds its identifier in the routing information
MC68824 USER'S MANUAL
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C-3