Edge-Core ECS4660-28F, Management Manual, Page: 539 / 2020

C
HAPTER
14
| Basic Administration Protocols
Ethernet Ring Protection Switching
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The RPL owner node detects a failed link when it receives R-APS
(SF - signal fault) messages from nodes adjacent to the failed link.
The owner then enters protection state by unblocking the RPL.
However, using this standard recovery procedure may cause a non-
EPRS device to become isolated when the ERPS device adjacent to it
detects a continuity check message (CCM) loss event and blocks the
link between the non-ERPS device and ERPS device.
CCMs are propagated by the Connectivity Fault Management (CFM)
protocol as described under "Connectivity Fault Management" on
page 548 . If the standard recovery procedure were used as shown
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.
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.
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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,
non -E RPS
A
non -E RPS
RPL
Owner
RPL
X X
b l ocked b l ocked
fau l t
B C D E F