Users Guide

Ethernet Ring Protection 1047
Ethernet Ring Protection Timers
The following timers are used by the ERP control process to reduce
protection switching occurrences.
Hold-off
When a link failure or defect occurs, the event will not be reported
immediately to protection switching if the configured hold-off timer value is
non-zero. Instead, the hold-off timer will be started. Upon the expiry of the
hold-off timer, the ERP control process checks whether a defect still exists
along the trail that started the timer. If it does, the defect will be reported to
protection switching. This is used by the underlying Ethernet layer to filter
out intermittent link faults.
Wait-to-Restore
When the fault condition is cleared, the traffic channel reverts after the
expiry of a Wait-to-Restore (WTR) timer when a fault condition is not active.
This timer is used to avoid toggling protection states in case of intermittent
defects. This timer is only used by the RPL owner when in revertive
operation.
Guard Timer
The guard timer prevents ring nodes from acting upon outdated R-APS
messages and prevents the possibility of forming a closed loop. This is used by
all nodes when a clearing condition occurs. This is the amount of time that an
ERP control process discards R-APS messages before being allowed to process
them.
It is used when a link is recovered, yet at the same time older messages are
still propagating around the ring with failure indications. The guard timer is
used whenever a ring node receives an indication that a local switching
request has cleared (such as, local clear SF, Clear).
Wait-to-Block
The Wait-to-block (WTB) timer is used when issuing clearing of Forced
Switch (FS) or Manual Switch (MS) commands. It is only used by the RPL
owner in a revertive operation before blocking RPL. This timer value is five
seconds longer than the guard timer and is not configurable.