Chapter 13
| Basic Administration Protocols
Ethernet Ring Protection Switching
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b. Then, after the operator issues the Clear command (Configure
Operation page) at the RPL Owner Node, this ring node blocks
the ring port attached to the RPL, transmits an R-APS (NR, RB)
message on both ring ports, informing the ring that the RPL is
blocked, and flushes its FDB.
c. The acceptance of the R-APS (NR, RB) message triggers all ring
nodes to unblock any blocked non-RPL which does not have an
SF condition. If it is an R-APS (NR, RB) message without a DNF
indication, all ring nodes flush their FDB. This action unblocks
the ring port which was blocked as result of an operator
command.
â–
Recovery for Manual Switching – A Manual Switch command is
removed by issuing the Clear command (Configure Operation page) at
the same ring node where the Manual Switch is in effect. The clear
command removes any existing local operator commands, and triggers
reversion if the ring is in revertive behavior mode.
The ring node where the Manual Switch was cleared keeps the ring
port blocked for the traffic channel and for the R-APS channel, due to
the previous Manual Switch command. This ring port is kept blocked
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.
The Ethernet Ring Node where the Manual Switch was cleared
continuously transmits the R-APS (NR) message on both ring ports,
informing that no request is present at this ring node. The ring nodes
stop transmitting R-APS (NR) messages when they accept an RAPS (NR,
RB) message, or when another higher priority request is received.
If the ring node where the Manual Switch was cleared receives an
R-APS (NR) message with a Node ID higher than its own Node ID, it
unblocks any ring port which does not have an SF condition and stops
transmitting R-APS (NR) message on both ring ports.
â–
Recovery with revertive mode is handled as follows:
a. The RPL Owner Node, upon reception of an R-APS (NR)
message and in the absence of any other higher priority
request, starts the WTB timer and waits for it to expire. While
the WTB timer is running, any latent R-APS (MS) message is
ignored due to the higher priority of the WTB running signal.
b. When the WTB timer expires, it generates the WTB expire
signal. The RPL Owner Node, upon reception of this signal,
initiates reversion by blocking the traffic channel on the RPL,
transmitting an R-APS (NR, RB) message over both ring ports,
informing the ring that the RPL is blocked, and flushes its FDB.
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