Allied Telesis X900-48FE, RAPIER I, AT-8900 Control Vlan, Data Vlan, Master Node, Transit Nodes

Ethernet Protection Switching Ring (EPSR) 3

The EPSR control VLAN, and its associated data VLANs, form a Ring Domain. Although a physical ring can have more than one domain, each domain must operate as a separate logical group of VLANs and must have its own master node. This means that several domains may share the same physical network, but must operate as logically separate VLAN groups.

The Control VLAN

The function of the control VLAN is to monitor the ring domain and maintain its operational functions. To do this it transmits and monitors operational healthcheck messages using EPSR healthcheck control frames. The control VLAN carries no user data.

Data VLAN

The data VLAN carries the user data around the ring. Several data VLANs can share a common control VLAN.

The Master Node

The master node controls the ring operation. It issues healthcheck messages at regular intervals from its primary port and monitors their arrival back at its secondary port, after they have circled the ring. Under normal operating conditions the master node’s secondary port is always in the blocking state to all data VLAN traffic. This is to prevent data loops forming within the ring. This port however, operates in the forwarding state for the traffic on the control VLAN. Loops do not occur on the control VLAN, because the control messages stop at the secondary port, having completed their path around the ring.

The Transit Nodes

The transit nodes operate as conventional Ethernet bridges, but with the additional capability of running the EPSR protocol. This protocol requires the transit nodes to forward the healthcheck messages from the master node, and respond appropriately when a ring fault is detected. The fault condition procedure is explained in the section, “Fault Detection and Recovery” on page 1-4.