Allied Telesis X900-48FE-N, AT-8948 Recovering from a Fault, Fault in a link or a transit node

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

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.

4.sending an EPSR Ring-Down-Flush-FDBcontrol 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

Page 5 AlliedWare™ OS How To Note: EPSR