System information

Troubleshooting Fiber Distributed Data Interface 5-69
Frame Format
As FDDI networks grow, the possibility of multiple ring failures grows. When two ring failures
occur, the ring is wrapped in both cases, effectively segmenting the ring into two separate rings that
cannot communicate with each other. Subsequent failures cause additional ring segmentation.
Optical bypass switches can be used to prevent ring segmentation by eliminating failed stations from
the ring. This is shown in Figure 5-6.
Figure 5-6 The Use of an Optical Bypass Switch
Critical devices such as routers or mainframe hosts can use another fault-tolerant technique called
dual homing to provide additional redundancy and help guarantee operation. In dual-homing
situations, the critical device is attached to two concentrators. One pair of concentrator links is
declared the active link; the other pair is declared passive. The passive link stays in backup mode
until the primary link (or the concentrator to which it is attached) is determined to have failed. When
this occurs, the passive link is automatically activated.
Frame Format
FDDI frame formats (shown in Figure 5-7) are similar to those of Token Ring.
Station 1
Station 3
Station 2
Optical bypass switch
“normal configuration”
Failed station
Station 4
A
A
B
A
B
B
AB
Station 1
Station 3
Station 2
Ring does
not wrap
Optical bypass switch
“bypassed configuration”
Station 4
B
A
B
A
B
A
AB