EAPS Single Ring Topology

The simplest EAPS configuration operates on a single ring.

This section describes how this type of EAPS configuration operates. Later sections describe more complex configurations.

An EAPS domain consists of one master node and one or more transit nodes (see the following figure), and includes one control VLAN and one or more protected VLANs.

A domain is a single instance of the EAPS protocol that defines the scope of protocol operation. A single logical EAPS domain typically exists on a given physical ring topology (fiber or copper).

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EAPS Operation

A protected VLAN is a user data VLAN that uses the ring for a protected connection between all edge ports. The protected VLAN uses 802.1q trunking on the ring ports and supports tagged and untagged edge ports.

One ring port of the master node is designated the master node‘s primary port (P), and another port is designated as the master node‘s secondary port (S) to the ring. In normal operation, the master node blocks the secondary port for all protected VLAN traffic, thereby preventing a loop in the ring. (The spanning tree protocol, STP, provides the same type of protection.) Traditional Ethernet bridge learning and forwarding database mechanisms direct user data around the ring within the protected VLANs.



Although primary and secondary ports are configured on transit nodes, both port types operate identically as long as the transit node remains a transit node. If the transit node is reconfigured as a master node, the configured states of the primary and secondary ports apply.
The control VLAN is a dedicated 802.1q tagged VLAN that is used to transmit and receive EAPS control frames on the ring. The control VLAN can contain only two EAPS ring ports on each node. Each EAPS domain has a unique control VLAN, and control traffic is not blocked by the master node at any time. The control VLAN carries the following EAPS control messages around the ring:
  • Health-check messages, which are sent from the master node primary port. Transit nodes forward health-check messages toward the master node secondary port on the control VLAN. When the master node receives a health check message on the secondary port, the EAPS ring is considered intact.

  • Link-down alert messages, which are sent from a transit node to the master node when the transit node detects a local link failure.

  • Flush-FDB messages, which are sent by the master node to all transit nodes when ring topology changes occur. Upon receiving this control frame, the transit node clears its MAC address forwarding table (FDB) and relearns the ring topology.

When the master node detects a failure, due to an absence of health-check messages or a received link-down alert, it transitions the EAPS domain to the Failed state and unblocks its secondary port to allow data connectivity in the protected VLANs.