Media Redundancy Protocol
The Media Redundancy Protocol is a protocol for fault-tolerant networks, as they are needed in critical automation applications. The availability is increased by inserting redundancy.
Principle of operation
MRP allows single failures in a simple ring topology to compensate. Since no meshed topologies are supported, MRP is deterministic and considerably simpler than RSTP.
The medium Redundancy Protocol ( MRP) based on a ring topology and guaranteed recovery times between 200 ms and 500 ms ( depending on configuration). MRP uses a redundancy manager (RM or MRM), which closes the ring. In normal operation, the redundancy manager checked by special test packets of the ring continuity. But he does not forward packets and thus prevent this endlessly circulate in the ring.
If one switch (SW) or a line, the transmitted test packets on a port at the other port will no longer be received. The redundancy manager forwards from now on to the packets in both directions and informs the switches on the topology change, so they do not give their packages on the dotted line, but send over the redundancy manager.
The redundancy manager is typically a logic function in a switch, is depicted here as a separate component.
Background
The data network standard Ethernet allows only one, so unique data path between any two nodes. By connecting multiple switches with cables, so can be built up line, star and tree structures. Falls from now for example, in a star topology, the switch in the center, all the other switches are isolated from one another.
Is a linear structure made up of switches, and the two switches are connected together at the ends of the line, then the ring structure is formed. Now there are two paths between any two participants who are connected to different switches. This is not acceptable. Therefore, a connection must be switched off the ring at the logical level. That is, the wiring is a ring structure, but in fact, the one line.
For this shutdown, there are various so-called protocols. In addition to the Rapid Spanning Tree Protocol RSTP are now available with the Media Redundancy Protocol the second standardized solution. If one of the other routes in the - physical - from the ring, the previously disconnected route is automatically activated. This can then, despite a link failure, all participants exchange data with each other more.
International Organization for Standardization
MRP is based on the HIPER-Ring, a protocol developed by Hirschmann and Siemens and 1999 submitted for ring redundancy. Corresponding products are on the market for some time.
Since April 2008, MRP is defined in the standard IEC 62439. With the normalization and the patents of both companies were made generally licensable: normal MRP nodes are royalty free, however, the redundancy manager must be licensed.
Objective of this standard are redundancy process with defined recovery times. The method should be independent of higher-level protocols and are suitable for real-time Ethernet mechanisms in IEC 61784 standard are described.
Bibliography
- IEC 62439 "Industrial communication networks: high availability automation networks" ( in the IEC Webstore )