Chapter 13
| Basic Administration Protocols
Ethernet Ring Protection Switching
– 480 –
In order to coordinate timing of protection switches at multiple layers, a hold-
off timer may be required. Its purpose is to allow, for example, a server layer
protection switch to have a chance to fix the problem before switching at a
client layer.
When a new defect or more severe defect occurs (new Signal Failure), this event
will not be reported immediately to the protection switching mechanism if the
provisioned hold-off timer value is non-zero. Instead, the hold-off timer will be
started. When the timer expires, whether a defect still exists or not, the timer
will be checked. If one does exist, 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outdated R-APS messages.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possible a timing anomaly where the RPL owner node receives an outdated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)
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