Web Management Guide-R05

Chapter 13 | Basic Administration Protocols
Ethernet Ring Protection Switching
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When non-ERPS device protection is enabled on an RPL owner node, it will
send non-standard health-check packets to poll the ring health when it
enters the protection state. It does not use the normal procedure of waiting
to receive an R-APS (NR - no request) message from nodes adjacent to the
recovered link. Instead, it waits to see if the non-standard health-check
packets loop back. If they do, indicating that the fault has been resolved,
the RPL will be blocked.
After blocking the RPL, the owner node will still transmit an R-APS (NR, RB -
ring blocked) message. ERPS-compliant nodes receiving this message flush
their forwarding database and unblock previously blocked ports. The ring
is now returned to Idle state.
◆ Holdoff Timer – The hold-off timer is used to filter out intermittent link faults.
Faults will only be reported to the ring protection mechanism if this timer
expires. (Range: 0-10000 milliseconds, in steps of 100 milliseconds)
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outdated R-APS messages.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.