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in the following figure, and node E detected CCM loss, it would send
an R-APS (SF) message to the RPL owner and block the link to node
D, isolating that non-ERPS device.
Figure 289: Non-ERPS Device Protection
A
B
When non-ERPS device protection is enabled on the ring, the ring
ports on the RPL owner node and non-owner nodes will not be
blocked when signal loss is detected by CCM loss events.
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receiving outdated R-APS messages.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
– 511 –
| Basic Administration Protocols
C
14
HAPTER
Ethernet Ring Protection Switching
C
D
blocked
fault
X
non-ERPS
non-ERPS
RPL
RPL
E
F
Owner
blocked
X
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