Each station receives all the span status change information
required to make protection switching decisions reliably, and fast.
Topology discovery provides a reliable and accurate means for all stations on a ring
to discover the topology of the stations on the ring, and any changes to that topology. It
also provides a mechanism for rapid detection of topology changes, and a mechanism
to convey additional station information to the ring.
The topology and protection protocol has the following features:
?– Responsive
?– Restoration in less than 50 ms
?– Quick dissemination of changes in the protection state
?– Robust
?– Support of a comprehensive protection hierarchy
?– Dynamic addition and removal of stations
?– Topology and protection frame loss tolerant
?– No need for a master or a management station
Figure 12.9 FDD calculation
FDD = FDD arrival class C - FDD arrival class A
S0 S2 S1 S3 S4 S2 S1 S3 S4 S0
As soon
as possible
Class A
FDD
Class C
FDD
Tail of congestion domain Tail of congestion domain
Resilient Packet Ring (RPR) 341
?– Assurance of topology image conversion for all stations
?– Context containment for strict order traffic
?– Flexible
?– Support of revertive and non-revertive operation
?– Closed and open ring topologies
?– Scalable to 255 stations
?– Means to share additional information between stations
?– Efficient
?– Requires insignificant ring traffic
?– Consumes minimal software execution time
?– Requires minimal hardware
The standard defines two protection mechanisms: Steer (mandatory) and Wrap
(optional)
Steer Steering stations direct unicast traffic onto ringlet0 or ringlet1 on a per destination
basis, to avoid failed spans.
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