Web Management Guide-R02

Chapter 13 | Basic Administration Protocols
Ethernet Ring Protection Switching
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Recovery for Protection Switching – A ring node that has one or more ring
ports in an SF (Signal Fail) condition, upon detecting the SF condition
cleared, keeps at least one of its ring ports blocked for the traffic channel
and for the R-APS channel, until the RPL is blocked as a result of ring
protection reversion, or until there is another higher priority request (e.g.,
an SF condition) in the ring.
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.
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Recovery with Revertive Mode – When all ring links and ring nodes
have recovered and no external requests are active, reversion is
handled in the following way:
a. The reception of an R-APS (NR) message causes the RPL Owner
Node to start the WTR (Wait-to-Restore) timer.
b. The WTR timer is cancelled if during the WTR period a higher
priority request than NR is accepted by the RPL Owner Node or is
declared locally at the RPL Owner Node.
c. When the WTR timer expires, without the presence of any 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.
d. The acceptance of the R-APS (NR, RB) message causes all 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.
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Recovery with Non-revertive Mode – In non-revertive operation, 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:
a. The RPL Owner Node does not generate a response on reception of
an R-APS (NR) messages.
b. When other healthy ring nodes receive the NR (Node ID) message,
no action is taken in response to the message.