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
.