Token Ring Switch Module User Guide
Page B-1
APPENDIX B
BRIDGING METHODS
B.1 OVERVIEW
Each port of the ATX can be configured for transparent (802.1d) bridging,
IBM source routing bridging, or source routing transparent bridging
(802.5M). Depending on network topology, it may be desirable to include
a mix of these methods within a single ATX.
The choice of bridging methods is determined both by end station
requirements and by other internetworking equipment.
Source routing end stations may use any of the three bridging methods.
Transparent end stations must use either transparent or SRT bridging.
When in doubt, transparent bridging is the easiest to configure and use.
If redundant links are employed along with IBM source routing bridges,
then the attached ATX port should be configured for source routing. This
will enable the mesh of bridges to derive a spanning tree suitable for
spanning tree explorer frames and for multicast packets.
If source routing is desired, and either Ethernet or FDDI is to be used as a
backbone between Token Rings, then the Ethernet or FDDI port should be
configured for SRT bridging. (SRT over Ethernet is not a standard, but is
available for use between multiple ATX chassis in backbone applications.
In this case, the “Ethernet” may actually be a microwave or satellite link
with an Ethernet-like interface.)
A common mixture of bridging modes may occur when Ethernet
segments and Token- Ring segments do not exchange data but share an
FDDI backbone. In this case, the Ethernets may be configured for
transparent bridging, the Token Rings for source routing, and the FDDI
backbone for SRT. (Don't infer from this example that SRT is the sum of
transparent and source routing bridging; it is a distinct third method).
The bridging method is dependent on the configuration of the bridge entry
and exit ports, and the value of the Routing Indicator (RII) bit in the
received frame. The following chart summarizes the interaction between
the bridging method.
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