Media Redundancy Protocol (MRP)

Media Redundancy Protocol Overview

What is it?

Media redundancy is primarily used to avoid single points of failure in industrial communication networks. If a failure occurs on a redundant structure, the network falls back to a secondary state in which communication is still viable, and repair can be made to restore the system to the previous fault-free state. Ethernet technology does not allow physical loops, as they cause packets to circulate endlessly and overload the network. This means providing media redundancy within an Ethernet network requires the use of a protocol that is able to monitor and resolve the physical loops introduced by redundant pathways. This protocol must ensure that, even with multiple physical pathways to any device, only one is activated at any one time and the remaining are in standby mode. This is achieved by: monitoring links, detecting interruptions, and switching to an alternative path in the event of failure as soon as possible. Media Redundancy Protocol (MRP) is a protocol for providing redundancy in Ethernet networks via a ring. MRP is specified for ring networks with up to 50 devices. It guarantees fully predictable switchover behavior.

Important concepts

  1. MRP node: A switch contained within a ring network monitored by MRP.
  2. Redundancy Domain: A ring controlled by MRP.
  3. Media Redundancy Manager (MRM): A single MRP node that monitors and controls the ring network and reacts to any failure. 
  4. Media Redundancy Client (MRC): Any node within the ring that is not an MRM.
  5. Ring port: The two designated ports on each MRP node that are physically connected to the ring topology.

How it works

Ring ports can be in one of three states:
  1. Disabled: all packets received by the port are dropped.
  2. Blocking: all packets received by the port are dropped, with the exception of MRP protocol packets.
  3. Forwarding: all packets received by the port are forwarded.
The ring  itself  can  be  in one of  three states: 
  1. Closed: both  ports  on  all nodes are  linked up, one port  on  the MRM is  blocking, all  others  are forwarding.
  2. Open: both  ports  on the MRM are  forwarding, either  because  one  of  the  MRCs  has  signalled a port  down,  or  the  MRM  failed  to  receive  its own  test  frames.
  3. Undefined:  this  is set  before  the  actual  state of the ring  is  known.
During  normal operation,  one of  the MRM’s  ring  ports  is  blocking, while  the other is  forwarding. This puts the network  in  a  closed  state, where  the physical  ring  topology  is  in  a ring  structure,  and at the logical level is  in  a  linear  structure.  The  MRM sends MRP_Test  frames  (at  a  predetermined interval)  out both its  ring ports.  These will be forwarded around the  ring by the MRCs until they again reach  the  MRM.

In the case of a transmission failure  (for example, a ring port  becomes disabled), the MRM will fail to receive  its test frames.  After a  predefined number of test  packets are lost,  it will set its  blocking  ring port  to  forwarding.  This  enables  the  secondary network  path,  and puts  the  network in  an  open state  where  both  MRM ring  ports  are  forwarding.



Veryx ATTEST™-CTS Media Redundancy Protocol (MRP) automated test suite provides Equipment Manufacturers, an easy and efficient solution for the verification of MRP in Industrial Ethernet Switches. ATTEST enables significant speeding up of testing cycles and reduces the “time-to-market”.

Veryx has devised about 111 test cases that comprehensively test for MRP compliance. These test cases have been grouped into 8 convenient test groups based on the IEC specifications for each category of functions.
ATTEST MRP test cases verify the MRP support for:
  1. Manager : MRP switch connected with other MRP clients in a ring
  2. Client: MRP switch connected with other MRP manager and other MRP clients in a ring.
A typical industrial Ethernet network showing a ring with MRP Manager, Client


IEC 62439: Industrial communication networks: high availability automation networks. – 65c, 62439/Ed 1.0, 2007-04-06

Functionalities covered:

  1. Applicant state machine
  2. MRP Manager - Ring Port states
  3. MRP Manager Protocol State Machine
  4. MRP Manager with multiple clients
  5. MRP Clients - Ring port states
  6. MRP Client Protocol State Machine
  7. MRP Clients with MRM and other MRCs
  8. Multiple MRP managers in a Ring
  9. MRP Frame Format verification