TABLE 122 Name
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Protection Hierarchy Acronym FS SF SD MS WTR IDLE Description A management directive that forces a span to be deactivated A signal failure that deactivates a span A signal degradation that can deactivate a span A management directive that can deactivate a scan A timer that improves stability in the presence of transient failures None of the above
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Forced Switch Signal Fail Signal Degrade Manual Switch Wait to Restore Idle
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P1 Span FS SF
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P2 Span FS, SF SF
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Resulting Protection Topology
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FS SF SD MS WTR SD MS WTR IDLE
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SD, MS, WTR, IDLE SD, MS, WTR, IDLE MS, WTR, IDLE WTR, IDLE IDLE SD MS WTR IDLE
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Figure 1212 Protection topologies
of a physical failure condition The glitches could be due to protection switching of RPR traffic by underlying SONET infrastructure, as shown in Figure 1213 As illustrated in Figure 1213, if the hold off timeout is omitted, the RPR will protect the services before SONET layer protection is completed Soon after the SONET layer completes its protection task, the RPR will detect that the failure has been removed, and will restore the original path for the services An unnecessary protection hit has been performed 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 1214 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
Resilient Packet Ring (RPR)
RPR station SONET ADM
Failure in the SONET ring
SONET protection fixes problem S1 S2 S3 S4 S1 S2 S3 S4
RPR over SONET without holdoff
255 Ringlet 0
macAddress macAddress My topoinfo
1 2 Ringlet 1
Figure 1214 Topology data base
from the specific station, and the ATD and LRTT frames are used exclusively to fill in additional information regarding each station
Context Containment Context containment assists in preventing duplication and reordering of strict mode traffic during topology changes Context containment is entered on detection of a passthrough, or when a span status changes Context containment is not exited until the topology is determined to be stable and valid and the local topology database is determined to match those of its reachable neighbors Each station calculates a topology checksum, unique to the ring, and uses the Topology Checksum (TC) frames to make its calculated topology checksum available to other stations in the ring During context containment all strict frames are discarded Stations receiving strict frames whose hops count does not match the expected hops counts for a station, as reflected in the topology data base, discard them In these cases, relaxed frames are not discarded
The Operations, Administration, and Maintenance (OAM) entity of RPR provides a set of control functions and indications to support fault management, and performance management The services provided by the RPR OAM are
Determine and validate connectivity between any two stations on the ring Determine and validate transit path operation for any service class Provide a mechanism to help in misorder prevention Provide performance monitoring parameters
Special control frames enable the detection and isolation of failures at the ring layer These frames can be used either during service provisioning or continuously to minimize the correction time of abnormal operation The OAM frame types supported are: Echo request/response, Flush and Organization specific
The client can request an echo request/response operation to a specified destination in order to check the reachability of that station An echo operation allows a frame to be inserted at one station in the ring, and an echo response returned by another station through the same or opposite ringlet Echo request/response frames can be assigned to any CoS
Flush The client can request to perform a flush operation A flush has the effect of clearing the selected ringlet of previously sourced traffic A flush is expected to be used when changing the ringlet selection by the client, to implement client controlled steer protection,
Resilient Packet Ring (RPR)
or for traffic engineering reasons A flush is a special control frame that is sent from a station to itself Flush operations prevent mis-order during steer protection The flush frames can be assigned to any CoS, with distinct flush action results, as follows:
ClassA Previously-sourced primary transit queue (PTQ) traffic is flushed ClassB or classC Previously-sourced PTQ and secondary transit queue (STQ) traffic is flushed