Allied Telesis AT-S63 - Manual (Page 195)

AT-S63 Management Software Features Guide
Section III: Snooping Protocols 195
Overview
Ethernet Protection Switching Ring is a feature found on selected Allied
Telesis products, such as the AT-8948 Series Gigabit Layer 3 Switches. It
offers an effective alternative to spanning tree based options when using
ring based topologies to create high speed resilient networks.
EPSR consists of a master node and a number of transit nodes in a ring
configuration. The master node monitors the health of the ring by
transmitting healthcheck messages from a primary port at regular intervals
over a control VLAN, and watching for the messages on a secondary port.
If the healthcheck messages fail to arrive, the master node commences
fault recovery of the ring by activating the secondary port so that
connectivity between the transit nodes is maintained through the master
node. When the integrity of the ring is restored, and the healthcheck
messages can again traverse the entire ring, the master switch returns the
secondary port to the blocking state.
Note
For background information and configuration examples of EPSR,
refer to the AlliedWare OS Software Reference Guide.
EPSR snooping gives the AT-9400 Switch the ability to function as a
transit node of a ring, but with restrictions, as explained in the next section.
The switch can forward healthcheck messages over the control VLAN
from the master node and respond appropriately when notified of a ring
fault by the master node.
The master node generates a variety of messages over the control VLAN
for monitoring the health of the ring and notifying the nodes of changes to
the ring’s status. Two of these messages are the Ring-Down-Flush-FDB
and Ring-Up-Flush-FDB messages. The first message notifies the nodes
of a ring fault condition and the second signals the reestablishment of the
ring.
The AT-9400 Switch and EPSR snooping react to these messages by
flushing the addresses learned on the two ring ports of the control VLAN
from the forwarding database, so that the switch can relearn the
addresses. These are the only two EPSR messages that EPSR snooping
can react to. It should be noted that EPSR snooping cannot generate any
EPSR messages itself.
To configure the AT-9400 Switch as a transit node you need to create the
control and data VLANs of the individual ring domains. As explained in the
EPSR chapter in the AlliedWare OS Software Reference Guide, several
domains can share the same physical network, but they must operate as
logically separate VLAN groups. For information on VLANs, refer to
Chapter 23, “Port-based and Tagged VLANs” on page 253.