How EPSR Works
Establishing a Ring
Once you have configured EPSR on the switches, the following steps complete the EPSR ring:
1.The master node creates an EPSR Health message and sends it out the primary port. This increments the master node’s Transmit: Health counter in the show epsr count command.
2.The first transit node receives the Health message on one of its two ring ports and, using a hardware filter, sends the message out its other ring port.
Note that transit nodes never generate Health messages, only receive them and forward them with their switching hardware. This does not increment the transit node’s Transmit: Health counter. However, it does increment the Transmit counter in the show switch port command.
The hardware filter also copies the Health message to the CPU. This increments the transit node’s Receive: Health counter. The CPU processes this message as required by the state machines, but does not send the message anywhere because the switching hardware has already done this.
3.The Health message continues around the rest of the transit nodes, being copied to the CPU and forwarded in the switching hardware.
4.The master node eventually receives the Health message on its secondary port. The master node's hardware filter copies the packet to the CPU (which increments the master node’s Receive: Health counter). Because the master received the Health message on its secondary port, it knows that all links and nodes in the ring are up.
When the master node receives the Health message back on its secondary port, it resets the Failover timer. If the Failover timer expires before the master node receives the Health message back, it concludes that the ring must be broken.
Note that the master node does not send that particular Health message out again. If it did, the packet would be continuously flooded around the ring. Instead, the master node generates a new Health message when the Hello timer expires.
