Web Management Guide-R04

Table Of Contents
Chapter 13
| Basic Administration Protocols
Ethernet Ring Protection Switching
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Only one RPL owner can be configured on a ring. If the switch is set as
the RPL owner for an ERPS domain, the west ring port is set as one end
of the RPL. If the switch is set as the RPL neighbor for an ERPS domain,
the east ring port is set as the other end of the RPL.
The east and west connections to the ring must be specified for all ring
nodes. When this switch is configured as the RPL neighbor, the east
ring port is set as being connected to the RPL.
Note that is not mandatory to declare a RPL neighbor.
Revertive – Sets the method of recovery to Idle State through revertive or non-
revertive mode. (Default: Enabled)
Revertive behavior allows the switch to automatically return the RPL from
Protection state to Idle state through the exchange of protocol messages.
Non-revertive behavior for Protection, Forced Switch (FS), and Manual
Switch (MS) states are basically the same. Non-revertive behavior requires
the RPL to be restored from Protection state to Idle state using the Clear
command (Configure Operation page).
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.
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.