Allied Telesis AlliedWare OS - Manual (Page 3)

Page 3 | AlliedWare™ OS How To Note: EPSR
How EPSR Works
How EPSR Works
EPSR operates on physical rings of switches (note, not on
meshed networks). When all nodes and links in the ring
are up, EPSR prevents a loop by blocking data transmission
across one port. When a node or link fails, EPSR detects
the failure rapidly and responds by unblocking the blocked
port so that data can flow around the ring.
In EPSR, each ring of switches forms an EPSR domain.
One of the domain’s switches is the master node and
the others are transit nodes. Each node connects to the
ring via two ports.
One or more data VLANs sends data around the ring,
and a control VLAN sends EPSR messages. A physical
ring can have more than one EPSR domain, but each
domain operates as a separate logical group of VLANs and
has its own control VLAN and master node.
On the master node, one port is the primary port and
the other is the secondary port. When all the nodes in
the ring are up, EPSR prevents loops by blocking the data
VLAN on the secondary port.
The master node does not need to block any port on the
control VLAN because loops never form on the control
VLAN. This is because the master node never forwards
any EPSR messages that it receives.
The following diagram shows a basic ring with all the
switches in the ring up.
EPSR Components
EPSR domain:
A protection scheme for an
Ethernet ring that consists of
one or more data VLANs and a
control VLAN.
Master node:
The controlling node for a
domain, responsible for polling
the ring state, collecting error
messages, and controlling the
flow of traffic in the domain.
Transit node:
Other nodes in the domain.
Ring port:
A port that connects the node
to the ring. On the master node,
each ring port is either the
primary port or the secondary
port. On transit nodes, ring
ports do not have roles.
Primary port:
A ring port on the master node.
This port determines the
direction of the traffic flow, and
is always operational.
Secondary port:
A second ring port on the
master node. This port remains
active, but blocks all protected
VLANs from operating unless
the ring fails. Similar to the
blocking port in an STP/RSTP
instance.
Control VLAN:
The VLAN over which all
control messages are sent and
received. EPSR never blocks this
VLAN.
Data VLAN
A VLAN that needs to be
protected from loops. Each
EPSR domain has one or more
data VLANs.
Data
VLA
N_
2
Data
VLA
N_
1
Contro
l VLA
N
Master
Node
Transit
Node
1
Transit
Node
4
Transit
Node
2
Data VLAN_1
Control VLAN
Primary Port
Transit
Node
3
epsr-basic-ring
Control VLAN
Data VLAN_2
P
S Secondary Port
Control VLAN is forwarding
Data VLAN is forwarding
End User Ports
Data VLAN_2
Control VLAN is forwarding
Data VLAN is blocked
Data VLAN_1
P S
End User Ports
End User Ports
End User Ports
End User Ports