C
HAPTER
14
| Basic Administration Protocols
Ethernet Ring Protection Switching
– 506 –
that defect will be reported to the protection switching mechanism. The
reported defect need not be the same one that started the timer.
◆ Guard Timer – The guard timer is used to prevent ring nodes from
receiving outdated R-APS messages. During the duration of the guard
timer, all received R-APS messages are ignored by the ring protection
control process, giving time for old messages still circulating on the ring
to expire. (Range: 10-2000 milliseconds, in steps of 10 milliseconds)
The guard timer duration should be greater than the maximum
expected forwarding delay for an R-APS message to pass around the
ring. A side-effect of the guard timer is that during its duration, a node
will be unaware of new or existing ring requests transmitted from other
nodes.
◆ WTB Timer – The Wait to Block (WTB) timer is used when clearing
Forced Switch (FS) and Manual Switch (MS) commands. As multiple FS
commands are allowed to co-exist in a ring, the WTB timer ensures that
clearing of a single FS command does not trigger re-blocking of the
RPL. When clearing an MS command, the WTB timer prevents the
formation of a closed loop due to possible a timing anomaly where the
RPL owner node receives an outdated remote MS request during the
recovery process.
When recovering from an FS or MS command, the delay timer must be
long enough to receive any latent remote FS or MS commands. This
delay timer called the WTB timer is defined to be 5 seconds longer than
the guard timer. This is enough time to allow a reporting ring node to
transmit two R-APS messages and allow the ring to identify the latent
condition.
This delay timer is activated on the RPL owner node. When the relevant
delay timer expires, the RPL owner node initiates the reversion process
by transmitting an R-APS (NR, RB) message. The delay timer, (i.e.,
WTR or WTB) is deactivated when any higher priority request preempts
this delay timer.
The delay timers (i.e. WTR and WTB) may be started and stopped by
the system. A request to start running the delay timer does not restart
the delay timer. A request to stop the delay timer stops the delay timer
and resets its value. The Clear command (Configure Operation page)
can be used to stop the delay timer.
◆ WTR Timer – The wait-to-restore timer is used to verify that the ring
has stabilized before blocking the RPL after recovery from a signal
failure. (Range: 5-12 minutes)
If the switch goes into ring protection state due to a signal failure, after
the failure condition is cleared, the RPL owner will start the wait-to-
restore timer and wait until it expires to verify that the ring has
stabilized before blocking the RPL and returning to the Idle (normal
operating) state.
◆ WTB Expire – The time before the wait-to-block timer expires.
◆ WTR Expire – The time before the wait-to-restore timer expires.
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