Fddi Fault Management - Cisco 7000 Series Hardware Installation And Maintenance Manual

Preparing Network Connections
Figure 2-22
SASs (Class B) typically attach to the primary ring through a concentrator, which provides
connections for multiple SASs. The concentrator ensures that a failure or power down of any SAS
does not interrupt the ring. SASs use one transmit port and one receive port to attach to the single
ring. DASs (Class A) have two physical ports, designated PHY A and PHY B, each of which
connects the station to both the primary and secondary rings. Each port is a receiver for one ring and
a transmitter for the other. For example, PHY A receives traffic from the primary ring, and PHY B
transmits to the primary ring.

FDDI Fault Management

The dual rings in an FDDI network provide fault tolerance. If a station on a dual ring shuts down or
fails, such as Station 3 in Figure 2-23, the ring automatically wraps (doubles back on itself) to form
a single contiguous ring. This removes the failed station from the ring, but allows the other stations
to continue operation. In Figure 2-23, the ring wraps to eliminate Station 3 and forms a smaller ring
that includes only Stations 1, 2, and 4.
Note
The stream is based on the signal flow on the primary ring. A station receives the primary signal from
its upstream neighbor, and transmits the primary signal to its downstream neighbor. For example,
Figure 2-23 shows the primary signal flow being transmitted from PHY B on Station 2 to PHY A on
Station 1 and from PHY B on Station 1 to PHY A on Station 4. Using Station 1 as a reference,
Station 2 is the upstream neighbor of Station 1, and Station 4 is the downstream neighbor of
Station 1.
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Typical FDDI Configuration with DAS, Concentrator, and SASs
FDDI
DAS Concentrator
SAS
Each station in a ring refers to its neighbor stations as upstream or downstream neighbors.
SAS SAS