Allied Telesis AT-S63 - Manual (Page 248)

Chapter 23: Ethernet Protection Switching Ring Snooping
248 Section III: Snooping Protocols
Figure 29. Double Fault Condition in EPSR Snooping
Now assume the link is reestablished between the switch and transit node.
At that point, the port on the transit node enters a preforwarding state in
which it forwards EPSR packets over the control VLAN to the AT-9400
Switch. However, the transit node does not forward traffic over the data
VLANs of the ring until it receives a “links up” message from the unit on
the other side of the repaired break, in this case the AT-9400 Switch.
Since EPSR snooping is not capable of generating EPSR messages, the
transit node does not receive the anticipated signal, and so the data
VLANs remain inactive. As a result, the AT-9400 Switch remains isolated
from the ring until the other break is repaired and the master switch sends
the EPSR Ring-Up-Flush-FDB message, which, in effect, initializes the
ring.
AT-9400 Switch
Transit Node
Transit Node
Master Node
AT-8948 Switch
P S