Media Redundancy Protocol (MRP) is a networking protocol designed to implement redundancy and recovery in a ring topology. MRP is designed to react deterministically on a single failure on a switch in the MRP ring.
In an MRP ring, according to IEC 62439-2, one of nodes in the network takes on the role of the media redundancy manager (MRM), and the other nodes are the redundancy clients (MRC). The MRM initiates and controls the ring topology to react to network faults by sending control frames on one ring port over the ring and receiving them from the ring over its other ring ports.
MRM and MRC ring ports support three status: disabled, blocked, and forwarding. Disabled ring ports drop all the received frames. Blocked ring ports drop all the received frames except the MRP control frames. Forwarding ring ports forward all the received frames.
During normal operation, the ring works in the Ring-Closed state. In this state, as a loop prevention, one of the MRM ring ports is blocked, while the other is forwarding. Conversely, both ring ports of all MRCs are forwarding. Loops are avoided because the physical ring topology is reduced to a logical stub topology.
In case of failure, the network works in the Ring-Open state. For instance, in case of failure of a link connecting two MRCs, the MRM sets both of its ring ports to the forwarding state; the MRCs adjacent to the failure have a blocked and a forwarding ring port; the other MRCs have both ring ports forwarding. So, in the Ring-Open status, the network logical topology is a stub.
MRP Rings
The customer will be deploying MRP rings in their substations for fast failover and ease of configuration.
Ring-Closed MRP Ring
This picture above shows an MRP ring in a closed condition. The MRM switch is the MRP Media Redundancy Manager and it is the designated switch that controls the ring and prevents the network loop from forming. “W” are the watchdog packets that transit the network much like RSTP BPDUs. If there is a line failure, the W frames alert the MRM to put its redundant port to forwarding.
For the blocked port on the MRM, only watchdog frames are allowed to pass, and not data frames.
Ring-Open MRP Ring
The figure below shows the ring in an open state with the MRM engaged.
MRP Ring Size
A ring of 50 switches is currently supported.
Media Redundancy Automanager
To configure a Media Redundancy Automanager (MRA), the node or nodes select an MRM by election and configured priority value.
The MRA role is not an operational MRP role like MRM or MRC. It is only an administrative temporary role at a device startup. A node must transition to the MRM role or the MRC role after startup, and the MRM is selected though the manager voting process.