Allied Telesis RAPIER I - Manual (Page 202)

4 Software Reference
Fault Detection and Recovery
EPSR uses two methods to detect and recover from outages in either a node or
a link within the ring. These methods are:
■ Master node polling fault detection
■ Transit node unsolicited fault detection
Master Node Polling Fault Detection
The master node issues healthcheck messages from its primary port as a means
of checking the condition of the EPSR network ring. These messages are sent at
regular periods, controlled by the hellotime parameter of the
create epsr
command on page 1-17 . A failover timer is set each time a healthcheck message
leaves the master node’s primary port. The timeout value for this timer is set
by the failover parameter of the
create epsr command on page 1-17 . If the
failover timer expires before the transmitted healthcheck message is received
by the master node’s secondary port, the master node assumes that there is a
fault in the ring, and implements its fault recovery procedures. Because this
detection method relies on a timer expiry, its operation is inherently slower
than the “transit node unsolicited detection method” described next.
Transit Node Unsolicited Fault Detection
This method relies on each transit node to detect a fault at its interface, and to
immediately notify the master node that a ring breakage has occurred. When a
transit node detects a connectivity loss, it immediately sends a “links down”
message over its good link. Because a link spans two nodes, both nodes will
send the “links down” message back to the master node. These nodes will also
change their state from “links up” to “links down,” and will change the state of
the port connecting to the broken link, from “forwarding” to “blocking.”