Multicast frames are sent in both directions, with the ttl
set to the number of stations to the defective span, on each ringlet.
As illustrated in Figure 12.10, station S2 normally sends to station S6 via the ringlet0.
After the fiber cut, S4 that detects the failure sends a protection message to all
stations in the ring. Based on this message, station S2 updates its topology database
and steers protected S6-destined traffic via the ringlet1. In flight frames, destined to
stations beyond the point of failure, are dropped at the edge. Steer protection has the
advantage that the traffic is routed through the optimal path even after a failure.
Figure 12.10 Steer protection
S1 S2 S3 S4 S5 S6
S1 S2 S3 S4 S5 S6
No edge in ring
Ring failure between
S3 and S4
342 Chapter 12
Wrap In addition to steering, RPR stations may support wrapping. Wrapping stations
direct traffic onto ringlet0 or ringlet1 on a per destination basis, regardless of failed
spans. An edge station wraps eligible frames that would otherwise be transmitted across
the edge.
As illustrated in Figure 12.11, station S2 normally sends to station S6 via the ringlet0.
After the fiber cut, station S2 still sends the traffic via the ringlet0 path.
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