Edge-Core ECS4810-12M Layer 2 Management Manual page 498

| Basic Administration Protocols
C 14
HAPTER
Ethernet Ring Protection Switching
A ring node that has one ring port in an SF condition and detects
the SF condition cleared, continuously transmits the R-APS (NR – no
request) message with its own Node ID as the priority information
over both ring ports, informing that no request is present at this
ring node and initiates a guard timer. When another recovered ring
node (or nodes) holding the link block receives this message, it
compares the Node ID information with its own Node ID. If the
received R-APS (NR) message has the higher priority, this ring node
unblocks its ring ports. Otherwise, the block remains unchanged. As
a result, there is only one link with one end blocked.
The ring nodes stop transmitting R-APS (NR) messages when they
accept an R-APS (NR, RB – RPL Blocked), or when another higher
priority request is received.
Recovery with Revertive Mode – When all ring links and ring
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nodes have recovered and no external requests are active,
reversion is handled in the following way:
The reception of an R-APS (NR) message causes the RPL
a.
Owner Node to start the WTR (Wait-to-Restore) timer.
The WTR timer is cancelled if during the WTR period a higher
b.
priority request than NR is accepted by the RPL Owner Node
or is declared locally at the RPL Owner Node.
When the WTR timer expires, without the presence of any
c.
other higher priority request, the RPL Owner Node initiates
reversion by blocking its traffic channel over the RPL,
transmitting an R-APS (NR, RB) message over both ring
ports, informing the ring that the RPL is blocked, and
performing a flush FDB action.
The acceptance of the R-APS (NR, RB) message causes all
d.
ring nodes to unblock any blocked non-RPL link that does not
have an SF condition. If it is an R-APS (NR, RB) message
without a DNF (do not flush) indication, all ring nodes flush
the FDB.
Recovery with Non-revertive Mode – In non-revertive operation,
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the ring does not automatically revert when all ring links and
ring nodes have recovered and no external requests are active.
Non-revertive operation is handled in the following way:
The RPL Owner Node does not generate a response on
a.
reception of an R-APS (NR) messages.
When other healthy ring nodes receive the NR (Node ID)
b.
message, no action is taken in response to the message.
When the operator issues the Clear command (Configure
c.
Operation page) for non-revertive mode at the RPL Owner
Node, the non-revertive operation is cleared, the RPL Owner
Node blocks its RPL port, and transmits an R-APS (NR, RB)
message in both directions, repeatedly.
Upon receiving an R-APS (NR, RB) message, any blocking
d.
node should unblock its non-failed ring port. If it is an R-APS
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