Users Guide

Ethernet Ring Protection 1039
Figure 26-2 shows a multi-ring topology. Here the major ring is fully formed,
while the sub-ring connects to the major ring at interconnection nodes. The
sub-ring is not fully formed, it is a partial ring in the shape of C and it does
not control the link on the major ring. Below are the two implementation
options for R-APS control channel of the sub-ring.
Without R-APS Virtual Channel
R-APS PDU’s processing on the sub-ring terminates at the interconnection
nodes. The RPL blocks data traffic but not R-APS PDUs in sub-ring.
With R-APS Virtual Channel
Here R-APS PDU’s in sub-rings are transmitted to the major ring through
interconnected nodes. R-APS channel blocking is the same as that of the
major ring. The RPL of the sub-ring blocks both R-APS PDU’s and data
traffic. This configuration is not supported.
Topology Change Notification
The topology change propagation process generates a signal to notify the ring
nodes of a major ring attached to a sub-ring about topology changes on the
sub-ring. This exists only on the ERP instance of sub-ring interconnection
nodes.
Once the major ring ERP instance is notified of the TCN by the sub-ring ERP
instance, the major ring instance will send out an R-APS message with a
special Flush Event sub-code. This message will go around the major ring
causing all the other ERP instances in the major ring to perform an FDB
flush. After that, MAC re-learning will occur and traffic along the protected
data VLANs can flow properly.
Protection Switching Triggers
Protection switching algorithm are triggered when:
• Initiated by manual command (for example, manual switch or forced
switch), if there are no requests with higher priority being in effect.
• A Signal Fail condition is reported on one of the ring links. Signal Failure
can be determined by CCM defects or a link down event.