User Manual UMN:CLI
V8102
205
7.7.19 Ring Automatic Protection Switching (R-APS)
The ITU-T Recommendation G.8032 specifies Ring Automatic Protection Switching (R-
APS) as a network protection mechanism for Ethernet ring topology to prevent frequent
operation of the protection switch due to an intermittent defect. An Ethernet ring consists
of R-APS nodes. Each node (switch) is configured as a Ring Protection Link (RPL) owner
node or a normal node. They exchange R-APS frames through ring ports. RPL-owner
node blocks one of the ring ports to ensure that there is no loop formed for the Ethernet
traffic. There is only one RPL-owner node in a ring. The normal nodes are responsible to
inform their ring’s owner node of Link failures/recovery. RPL is a single ring topology to be
controlled by R-APS mechanism.
Both RPL-owner node and normal node have a west and east port for R-APS operation.
Firstly, you should specify RPL ports, which are directly connected to the RPL link within
an Ethernet ring. One of ring ports of RPL-owner node is blocked as unused link for traffic
while it runs without the link failure detection. You should configure a west or east port to
be blocked.
R-APS Messages
There are three types of R-APS messages of concern to the RPL-owner node-Normal
node interaction in ring as shown below:
• Normal Node messages
The following messages are sent by the normal nodes to inform RPL-owner node of
their link changes.
– Signal Fail (SF): A normal node sends Signal Fail fames detecting its link failure.
– No Request (NR): A normal node sends No Request frames detecting its link
recovery.
• RPL-owner Node messages
An RPL-owner node is in charge of protecting the Ethernet ring. It sends periodic No
Request frames to normal nodes and receives Signal fail/No Request frame from
those nodes to detect the link failure or recovery.
– Signal Fail: A RPL-owner node sends Signal Fail fames detecting its link failure.
– No Request and RPL Blocked: This is used to inform the normal nodes of re-
blocking status of its RPL port caused by link recovery.
R-APS implementation of the V8102 has the following restrictions, so you should keep in
mind those before configuring R-APS.
• R-APS can not be configured with STP. For enabling R-APS in the system, STP
might be manually disabled.
• A west and east port number of RPL-owner node should be different.
• R-APS mechanism should be used for Ethernet Ring topology only.
If the link failure occurs, the nodes adjacent (Node A & B) to the failure detect their state
and send Signal Fail (SF) frames. If an intermediate node (Node C) between RPL-owner
node and a node adjacent to link failure receives SF frame, it starts to perform Forward-
ing Database (FDB) Flushing. FDB Flushing consists in erasing in the forwarding data-
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