HP 200 Series Services And Applications page 242

Shining a Light on FDDI
How FDDI Works
Optical Bypass
Optical bypass can be used for fault tolerance to prevent ring segmenta-
tion. Optical bypass switches maintain connectivity of the FDDI ring in the
absence of power or during fault conditions in a station. Stations bypassed
by optical bypass switches are effectively removed from the ring. Figure 5
shows an inoperative station switched out by an optical bypass switch.
Figure 5. DAS with Optical Bypass Switch
Optical bypass switches are recommended when designing a multi-
station FDDI backbone. In a fault condition, a DAS will cause the ring to
wrap over the secondary ring, isolating the bad section of the ring. If this
happens to more than any single section of the FDDI ring, the network is
sectioned into two independent rings. Stations that have an optical bypass
device are physically switched out when a fault condition occurs. They do
not prevent the network from segmenting in a cable fault condition.
3-48
Failed Station
DAS
Port Port
A B
Ring does
not wrap
Primary
Secondary
B A
Port Port
DAS
Optical Bypass
Switch