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Chapter 13
| Basic Administration Protocols
Ethernet Ring Protection Switching
d.
Upon receiving an R-APS (NR, RB) message, any blocking node
should unblock its non-failed ring port. If it is an R-APS (NR, RB)
message without a DNF indication, all ring nodes flush the FDB.
Recovery for Forced Switching – A Forced Switch command is removed
■
by issuing the Clear command (Configure Operation page) to the same
ring node where Forced Switch mode 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 Forced Switch was cleared continuously
transmits the R-APS (NR) message on both ring ports, informing other
nodes 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 Forced 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 over both ring ports.
Recovery with revertive mode is handled as follows:
■
a.
The reception of an R-APS (NR) message causes the RPL Owner
Node to start the WTB timer.
b.
The WTB timer is cancelled if during the WTB 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 WTB timer expires, in the absence of any other higher
priority request, the RPL Owner Node initiates reversion by
blocking the 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 flushes the FDB.
d.
The acceptance of the R-APS (NR, RB) message causes all ring
nodes to unblock any blocked non-RPL that 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 a result of an operator
command.
Recovery with non-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 does
not perform any action.
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.
– 428 –
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