Allied Telesis AT-S63 - Manual (Page 191)

AT-S63 Management Software Features Guide
Section III: Snooping Protocols 191
Restrictions
EPSR snooping has three important restrictions. All the restrictions are
related to control EPSR messages and the fact that EPSR snooping can
not generate these messages.
The AT-9400 Switch cannot fulfill the role of master node of a ring because
EPSR snooping does not generate EPSR control messages. That function
must be assigned to another Allied Telesis switch that supports EPSR,
such as the AT-8948 Fast Ethernet Layer 3 Switch. (For a list of Allied
Telesis products that support EPSR, refer to the company’s web site or
contact your sales representative.)
The second restriction is EPSR snooping does not support the transit node
unsolicited method of fault detection. When a break occurs in a ring, the
transit nodes on either side of the break can notify the master node by
sending a “links down” message over the control VLAN of the ring. This
method of fault detection and notification can be a faster way for the
master node to become aware of a problem than with the healthcheck
message. However, since EPSR snooping can not generate the “links
down” message, the AT-9400 Switch can not initiate this type of fault
notification.
The final restriction of EPSR snooping concerns how the switch responds
in the unlikely event it becomes isolated by disruptions in the ring on either
side of it. Because the switch can not generate the EPSR “links up”
message, a failure of a ring at two or more places may leave the AT-9400
Switch and EPSR snooping isolated until all of the breaks have been
repaired and the entirety of the ring is restored.
The problem is illustrated in Figure 17. The example is a ring of four
nodes: a master switch, two transit nodes, and the AT-9400 Switch
running EPSR snooping. The ring has a control VLAN along with one or
more data VLANs. The AT-9400 Switch is isolated from the ring because
of two breaks, one between it and the master node and another between it
and a transit node.