C
HAPTER
14
| Basic Administration Protocols
Ethernet Ring Protection Switching
– 505 –
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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 514. 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-ERPS
A
non-ERPS
RPL
Owner
RPL
XX
blocked blocked
fault
BCDEF
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