Cisco ASR 900 Series Configuration Manual page 58

Protection Switching Functionality in a Single Link Failure and Recovery
5 All Ethernet ring nodes receiving an R-APS SF message perform the FDB flush. When the RPL owner
node G and RPL neighbor node A receive an R-APS SF message, the Ethernet ring node unblocks its end
of the RPL and performs the FDB flush.
6 All Ethernet ring nodes receiving a second R-APS SF message perform the FDB flush again; the additional
FDB flush is because of the node ID and BPR-based configuration.
7 R-APS SF messages are detected on the Ethernet Ring indicating a stable SF condition. Further R-APS
SF messages trigger no further action.
The following figure illustrates the steps taken in a revertive operation in a single-link failure.
Figure 10: Single-Link Failure Recovery (Revertive Operation)
The following sequence describes the steps followed in the single-link failure revertive (recovery) operation:
1 A link operates in the stable SF condition.
2 Recovery of link failure occurs.
3 Ethernet ring nodes C and D detect clearing of the SF condition, start the guard timer, and initiate periodic
transmission of the R-APS No Request (NR) messages on both ring ports. (The guard timer prevents the
reception of R-APS messages.)
4 When the Ethernet ring nodes receive an R-APS NR message, the node ID and BPR identifier pair of a
receiving ring port is deleted and the RPL owner node starts the Wait-to-Restore (WTR) timer.
5 When the guard timer expires on Ethernet ring nodes C and D, the nodes may accept the new R-APS
messages, if any. Ethernet ring node D receives an R-APS NR message with a higher node ID from Ethernet
ring node C, and unblocks its nonfailed ring port.
LAN Switching Configuration Guide Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
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ITU-T G.8032 Ethernet Ring Protection Switching