Specifications

B.5 Physical Topologies
B–8 FDDI Overview
B.5 Physical Topologies
B.5.1 Topology Types
Although all FDDI networks are logical rings, the following physical topology types
can be created:
• Dual Rings
• Trees
• Dual Ring of Trees
B.5.2 Dual Ring Topologies
Dual Ring topologies (as shown in Figure B–1) are created by connecting only Dual
Attachment Stations. The A port of one DAS station connects to the B port of the
next DAS station, forming a physical connection between the two stations. The con-
nections can continue until up to 500 stations are included in the dual ring.
• Independent Rings
Because there are two physical links per physical connection (see Figure B–
2), two operating links are created between each DAS station when the ring
is formed. This design allows for two independent rings, the primary ring
and the secondary ring, to be formed.
The Primary ring enters the DAS station at the A port, and exits the DAS sta-
tion at the B port. The Secondary ring enters the DAS station at the B port
and exits the DAS station at the A port.
• Backup Benefit
In general, all data traffic between stations is carried on the primary ring and
the secondary ring is idle during this time. The FDDI standards actually
allow both rings to be used for carrying data traffic, but in practice this is
rarely done. The common practice is to use the secondary ring as a backup
for the primary ring.
A benefit of having dual rings is that, in case of a failure, the secondary ring
can be used to patch a failure affecting the primary ring. If a station detects
that one of its A or B ports has failed, it wraps the primary and secondary
rings together, thus restoring operation to the other stations in the ring.
Wrapping occurs very quickly, usually in well under a half a second.