If the hold off timeout is set to allow the SONET layer to complete protection, then
no RPR protection is implemented and the additional hit is avoided.
The RPR MAC builds a topology data base that is partitioned into multiple components:
ring level information, local station information, and per station information. The information
used to configure the values in the topology data base comes from a variety of sources
such as: TP frames, ATD frames, LRTT frames, and topology checksum (TC) frames.
Figure 12.14 shows an example of a data base built by an RPR station. The TP frames
are used to position the stations in the table, according to their hop count distance
Figure 12.12 Protection topologies
S1 S2 S3 S4 S5 S6
S1 S2 S3 S4 S5 S6
P1
P1
S1 S2 S3 S4 S5 S6
S1 S2 S3 S4 S5 S6
SD
MS
WTR
IDLE
SD
MS
WTR
IDLE
FS
SF
SD
MS
WTR
SD, MS, WTR, IDLE
SD, MS, WTR, IDLE
MS, WTR, IDLE
WTR, IDLE
IDLE
FS
SF
FS, SF
SF
P1 Span P2 Span Resulting Protection Topology
Resilient Packet Ring (RPR) 345
Figure 12.13 RPR over SONET without holdoff
S1 S2 S3 S4 S1 S2 S3 S4
S1 S2 S3 S4
Failure in the
SONET ring
SONET
protection
fixes
problem
RPR station
SONET ADM
Figure 12.
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