Web Management Guide-R04

Chapter 13 | Basic Administration Protocols
Ethernet Ring Protection Switching
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When a ring is under an FS condition, and the node at which an FS
command was issued is removed or fails, the ring remains in FS
state because the FS command can only be cleared at node where
the FS command was issued. This results in an unrecoverable FS
condition.
When performing a maintenance procedure (e.g., replacing,
upgrading) on a ring node (or a ring link), it is recommended that FS
commands be issued at the two adjacent ring nodes instead of directly
issuing a FS command at the ring node under maintenance in order to
avoid falling into the above mentioned unrecoverable situation.
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Manual Switch – Blocks specified ring port, in the absence of a failure or an
FS command. (Options: West or East)
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A ring with no request has a logical topology with the traffic channel
blocked at the RPL and unblocked on all other ring links. In this
situation, the Manual Switch command triggers protection switching as
follows:
a. If no other higher priority commands exist, the ring node, where a
manual switch command was issued, blocks the traffic channel and
R-APS channel on the ring port to which the command was issued,
and unblocks the other ring port.
b. If no other higher priority commands exist, the ring node where the
manual switch command was issued transmits R-APS messages
over both ring ports indicating MS. R-APS (MS) message are
continuously transmitted by this ring node while the local MS
command is the ring node’s highest priority command (see
Table 34 on page 506). The R-APS (MS) message informs other ring
nodes of the MS command and that the traffic channel is blocked
on one ring port.
c. If no other higher priority commands exist and assuming the ring
node was in Idle state before the manual switch command was
issued, the ring node flushes its local FDB.
d. A ring node accepting an R-APS (MS) message, without any local
higher priority requests unblocks any blocked ring port which does
not have an SF condition. This action subsequently unblocks the
traffic channel over the RPL.
e. A ring node accepting an R-APS (MS) message, without any local
higher priority requests stops transmitting R-APS messages.
f. A ring node receiving an R-APS (MS) message flushes its FDB.
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Protection switching on a manual switch request is completed when
the above actions are performed by each ring node. At this point, traffic
flows around the ring are resumed. From this point on, the following
rules apply regarding processing of further manual switch commands: