Station S6
does not strip frames from the opposite ringlet of that indicated in the wrapped frame
(ringlet1 in this case) to avoid frame mis-ordering during the protection event. In flight
frames, destined to stations beyond the point of failure, are wrapped at the edge.
Wrap protection is usually faster that steer protection, because the protection decision
(wrap traffic) is local to the stations detecting the failure (S3 and S4 in Figure 12.11).
Another advantage of wrapping is that there is no need to duplicate multicast frames,
since all stations remain reachable trough both ringlets.
Steer and wrap stations can not coexist in a ring, if a mismatch is detected an alarm
condition is declared. On the other hand, by using a special bit in the RPR header, a
ring that is configured for wrap protection can allow the client to steer specific packet
flows using the Selective Wrap Independent Steer (SWIS) method.
SWIS The RPR frame includes a wrap eligibility (we) bit. During a span failure, wrapping
stations wrap frames only if we is set, if we is clear frames are discarded at the
edge of the failure. In a wrapping ring, a client may specifically request that a frame be
sent by the MAC with the we clear, and by manipulating the parameters provided by
the client to the MAC for each transmit frame, the client can steer these frames during
a failure condition.
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