Industrial Managed
Ethernet Switch – EH9711
User Manual
Page
31
of
223
(a)
(b)
Figure 2.18 An Example of Ring Topology (a) Major Ring, and (b) Sub-Ring (Example made on EH7520)
An ERPS ring consists of interconnected Layer 2 switching devices configured with the same control VLAN. An ERPS ring
can be a major ring or a sub-ring, as shown in Figure 2.18. By default, an ERPS ring is a major ring. The
major ring
is a
closed ring, whereas a
sub-ring
is a non-closed ring. The major ring and sub-ring can be configured through
type
field. On the
network shown in Figure 2.18, switch EH9711A to EH9711C via EH9711B and EH9711D constitute a major ring, and switch
EH9711E through switch EH9711F constitute a sub-ring.
In the Ethernet ring, loops can be avoided by guaranteeing that traffic may flow on all but one of the ring links at any time.
This particular link is called
Ring Protection Link (RPL)
. A control message called
Ring Automatic Protection Switch (R-
APS)
coordinates the activities of switching on/off the RPL. Under normal conditions, this link is blocked by the designated
Ethernet Ring Node called
RPL Owner Node
to ensure that there is no loop formed for the Ethernet traffic. The node at the
other end of the RPL is known as
RPL Neighbor Node
. In case an Ethernet ring failure occurs, the RPL Owner Node will be
responsible for unblocking its end of the RPL to allow RPL to be used as a backup link. The RPL is the backup link when one
link failure occurs. Other ring ports called common port will help monitoring the status of the directly connected ERPS link
and send RAPS PDUs to notify the other ports of its link status changes.
In case that users do not want their clients to detect the fault and would like sometimes to rectify the problem, users may use
the Holdoff timer. If the fault occurs, the fault is not immediately sent to ERPS until the
Holdoff timer
expires.
If an RPL owner port is unblocked due to a link or node recovery after its faulty, the involved port may not be changed to Up
state immediately since it may cause network flapping. To prevent this problem, in
revertive
switching, the node where the
RPL owner port resides starts the
wait to restore (WTR) timer
after receiving a
RAPS No Request (NR)
message. If the
node receives a
RAPS Signal Fail (SF)
message before the timer expires, it will terminate the WTR timer. Otherwise, the RPL
owner will block its own port, and send out RAPS (no request or NR, root blocked or RB) messages to inform the other nodes
of the link or node recovery and starts the
Guard timer
. Before the Guard timer expires, other nodes do not process any RAPS
(NR) messages to avoid receiving out-of-date RAPS (NR) messages. After the
Guard timer
expires, if the other nodes still
receive RAPS (NR) messages, the nodes set their recovered ports on the ring to the Forwarding state. In
non-revertive
switching, the WTR timer is not started, and the original faulty link is still blocked.
ERPSv1
supports only revertive switching.
ERPSv2
supports both revertive and non-revertive switching.
Control messages of each ERPS ring (e.g., R-APS PDUs) are transmitted through a configuration of
a control VLAN
. For an
ERPS ring that is already configured a control VLAN, when users add a port to the ERPS ring, the port is automatically added
to the control VLAN. Different ERPS rings cannot be configured with the same control VLAN ID. The control VLAN must
be mapped to an
Ethernet Ring Protection (ERP) instance
, so that ERPS forwards or blocks the VLAN packets based on
blocking rules, protecting the ring network from broadcast storms.
shows the ERPS Configuration webpage. Table 2.13 summarizes the descriptions of columns in EPRS
Configuration’s table.
