Cisco 7000 Hardware Installation And Maintenance page 104

Preparing Network Connections
Figure 2-24
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 2-24 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.
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 2-24 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.
2-100 Cisco 7000 Hardware Installation and Maintenance
DAS Station Failure and Ring Recovery Example
Station 4
A
MAC
B
Each station in a ring refers to its neighbor stations as upstream or downstream neighbors.
Station 1
MAC
A
B
B
A
A B
Failed station
Station 3
Station 2
MAC
Ring wrap