Tripp Lite NGI-M05-C1, Owner'S Manual, Page: 128 / 254

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6.8. ERPS
The ITU-T G.8032 E thernet R ing P rotection S witching feature implements protection
switching mechanisms for Ethernet layer ring topologies. This feature uses the G.8032
Ethernet Ring Protection (ERP) protocol, defined in ITU-T G.8032, to provide
protection for Ethernet traffic in a ring topology, while ensuring that no loops are within
the ring at the Ethernet layer. The loops are prevented by blocking traffic on either a
predetermined link or a failed link.
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,