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Page 5 | AlliedWare™ OS How To Note: EPSR
How EPSR Works
Detecting a Fault
EPSR uses a fault detection scheme that alerts the ring
when a break occurs, instead of using a spanning tree-
like calculation to determine the best path. The ring
then automatically heals itself by sending traffic over a
protected reverse path.
EPSR uses the following two methods to detect when
a transit node or a link goes down:
• Master node polling fault detection
To check the condition of the ring, the master
node regularly sends Health messages out its
primary port, as described in "Establishing a
Ring" on page 4. If all links and nodes in the ring are
up, the messages arrive back at the master node on
its secondary port.
This can be a relatively slow detection method,
because it depends on how often the node sends
Health messages.
Note that the master node only ever sends Health
messages out its primary port. If its primary port
goes down, it does not send Health messages.
• Transit node unsolicited fault detection
To speed up fault detection, EPSR transit nodes
directly communicate when one of their interfaces
goes down. When a transit node detects a fault at
one of its interfaces, it immediately sends a Link-
Down message over the link that remains up. This
notifies the master node that the ring is broken and
causes it to respond immediately.
Recovering from a Fault
Fault in a link or a transit node
When the master node detects an outage somewhere
in the ring, using either detection method, it restores
traffic flow by:
1. declaring the ring to be in a Failed state
2. unblocking its secondary port, which enables data
VLAN traffic to pass between its primary and
secondary ports
3. flushing its own forwarding database (FDB) for the
two ring ports
4. sending an EPSR Ring-Down-Flush-FDB control message to all the transit nodes, via
both its primary and secondary ports
The transit nodes respond to the Ring-Down-Flush-FDB message by flushing their
forwarding databases for each of their ring ports. As the data starts to flow in the ring’s
Master Node States
Complete:
The state when there are no link or
node failures on the ring.
Failed:
The state when there is a link or
node failure on the ring. This state
indicates that the master node
received a Link-Down message or
that the failover timer expired before
the master node’s secondary port
received a Health message.
Transit Node States
Idle:
The state when EPSR is first
configured, before the master node
determines that all links in the ring
are up. In this state, both ports on
the node are blocked for the data
VLAN. From this state, the node can
move to Links Up or Links Down.
Links Up:
The state when both the node’s ring
ports are up and forwarding. From
this state, the node can move to
Links Down.
Links Down:
The state when one or both of the
node’s ring ports are down. From this
state, the node can move to Pre-
forwarding
Pre-forwarding:
The state when both ring ports are
up, but one has only just come up and
is still blocked to prevent loops. From
this state, the transit node can move
to Links Up if the master node blocks
its secondary port, or to Links Down
if another port goes down.