Tripp Lite NGI-S04C2 Owner's Manual page 36

The Ethernet ring protection functionality includes the following:
• Loop avoidance
• The use of learning, forwarding, and Filtering Database (FDB) mechanisms
Loop avoidance in an Ethernet ring is achieved by guaranteeing that, at any time, traffic
may flow on all but one of the ring links. This particular link is called the ring protection
link (RPL) and under normal conditions this ring link is blocked, i.e., not used for
service traffic. One designated Ethernet ring node, the RPL owner node, is responsible to
block traffic at one end of the RPL. Under an Ethernet ring failure condition, the RPL
owner node is responsible for unblocking its end of the RPL, unless the RPL has failed,
allowing the RPL to be used for traffic. The other Ethernet ring node adjacent to the RPL,
the RPL neighbor node, may also participate in blocking or unblocking its end of the
RPL.
The Ethernet rings could support a multi-ring/ladder network that consists of conjoined
Ethernet rings by one or more interconnection points. The protection switching
mechanisms and protocol defined in this Recommendation shall be applicable for a
multi-ring/ladder network, if the following principles are adhered to:
R-APS channels are not shared across Ethernet ring interconnections;

on each ring port, each traffic channel and each R-APS channel are controlled (e.g.,

for blocking or flushing) by the Ethernet ring protection control process (ERP
control process)of only one Ethernet ring;
Each major ring or sub-ring must have its own RPL.

In an Ethernet ring, without congestion, with all Ethernet ring nodes in the idle state (i.e.,
no detected failure, no active automatic or external command and receiving only "NR,
RB" R-APS messages), with less than 1200 km of ring fiber circumference and fewer
than 16 Ethernet ring nodes, the switch completion time (transfer time as defined in
[ITU-T G.808.1]) for a failure on a ring link shall be less than 50ms.
The ring protection architecture relies on the existence of an APS protocol to coordinate
ring protection actions around an Ethernet ring.
The Switch supports up to six rings.
Guard timer -- All ERNs use a guard timer. The guard timer prevents the possibility of
forming a closed loop and prevents ERNs from applying outdated R-APS messages. The
guard timer activates when an ERN receives information about a local switching request,
such as after a switch fail (SF), manual switch (MS), or forced switch (FS). When this
timer expires, the ERN begins to apply actions from the R-APS it receives. This timer
cannot be manually stopped.
Wait to restore (WTR) timer -- The RPL owner uses the WTR timer. The WTR timer
applies to the revertive mode to prevent frequent triggering of the protection switching
due to port flapping or intermittent signal failure defects. When this timer expires, the
RPL owner sends a R-APS (NR, RB) through the ring.
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