Cisco 7000 Series Hardware Installation And Maintenance Manual page 95

Figure 2-23 DAS Station Failure and Ring Recovery Example
Station 4
MAC
Ring wrap
A second failure could cause the ring to wrap in both directions from the point of failure, which
would segment the ring into two separate rings that could not communicate with each other. For
example, if Station 1 in Figure 2-23 fails after Station 3 fails, Stations 2 and 4 will each be isolated
because no path for communication exists between them. Subsequent failures cause additional
segmentation.
Optical bypass switches avoid segmentation by eliminating failed stations from the ring. During
normal operation, an optical bypass switch allows the light signal to pass directly through itself
uninterrupted. When a station with a bypass switch fails, the bypass switch reroutes the signal back
onto the ring before it reaches the failed station, so the ring does not have to wrap back on itself.
Figure 2-24 shows an optical bypass switch installed at Station 1. In the normal configuration shown
on the left, Station 1 is functioning normally, so the optical bypass switch appears transparent. The
switch essentially allows the signals to pass through it without interruption. However, if Station 1
fails, the optical bypass switch enables the bypassed configuration shown on the right in Figure 2-24.
The bypass switch reroutes the light signal by intercepting it before it reaches the failed Station 1
and sends it back out to the ring. This allows the signal to maintain its existing path and direction
without wrapping back on itself. However, stations that are operating normally repeat the signal
when sending it back out to the ring. Optical bypass switches do not repeat or drive the signal; they
just allow the signal to pass through them. Therefore, significant signal loss can occur when the
downstream neighbor, the next station on the ring, is far away.
Station 1
MAC
A
B
A
B
A B
Station 3
Preparing Network Connections
Station 2
B
MAC
A
Failed station
Preparing for Installation 2-35
Ring wrap