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Media Redundancy Protocol by ALE : What is it? What are the benefits?

Do you need to deliver a network solution inside a power grid, factory, or any other industrial facility?  Are you […]

Post on 01.06.2021 by Slaven
Digital Age Network Network management Trends

Do you need to deliver a network solution inside a power grid, factory, or any other industrial facility?  Are you looking for a high-availability network based on hardened Ethernet switches designed to operate in the harshest industrial environments and severe temperatures? Then, look no further than deploying an industrial network built upon the versatile OmniSwitch 6865 and OmniSwitch 6465 product lines running Media Redundancy Protocol (MRP).

MRP is a standard-based recovery protocol described in IEC 62439-2 standard applicable to high-availability automation networks with ring topologies. Its main functionality is to react deterministically, with predictable recovery mechanisms and recovery times, in case of a single switch or link failure in networks. As a standard based protocol interoperability among different vendors running MRP, it should allow a smooth expansion of the existing network or straightforward replacement of incumbent vendor. ALE ensures MRP interoperability with Hirschman switches as a part of the standardized Software Quality Assurance (SQA) service testing process.

MRP protocol defines predictable mechanisms of network recovery by assigning different “roles” and related functionalities to the switches in the ring. Media Redundancy Manager (MRM) role is assumed by a single switch in the ring.  Its tasks are to observe the ring topology by sending MRP test frames at a configured time period in both directions of the ring. It also controls the ring topology by keeping one ring port in forwarding while the other ring port is in blocked state if MRP test frames are received by another ring port. Keeping one ring port of MRM in blocked state for data plane frames prevents formation of a Layer 2 loop in the ring.  All other switches in the ring will assume Media Redundancy Client (MRC) role. The main tasks of MRCs, besides forwarding MRP test frames sent by MRM, are to detect link changes on its own ring ports and to signal those changes towards MRM. In case there is no link or switch failure in the network, the ring is closed.

MRP Ring blocked
Fig 1. MRP Ring blocked

 

In case of failure in the network, MRM will discover it. The first scenario is when it does not receive its own test frames through the ring within the pre-defined period of time. The second way would be by receiving the signal message from MRCs local to the failure, in the case it is supported with ALE switches. Upon discovery of the failure, MRM will start forwarding on both ring ports allowing for full connectivity within the ring as long as failure is not recovered.

Fig 2. MRP ring open
Fig 2. MRP ring open

 

When calculating a convergence time in case of failure in the ring, there are many parameters that must be considered. Some of them are related to MRM configured values for intervals of sending test frames and count for monitoring test frames.  The others are not directly related to MRM functionality such as the number of switches in the ring (standard number of switches must not exceed 50 per IEC 62439-2), forwarding time of each device, delay the frame experience while traveling through the network… Taking all parameters into account, the standard provides guidelines for convergence times (from 10 to 500 ms) but the correct times could be defined upon testing.

It is worth mentioning that ALE switches also support redundant interconnection topology if design requires one.

To conclude, MRP is supported starting AOS 8.7R2 on product lines of hardened switches OS6865 and OS6465. Within these product lines, there is a variety of fanless switches that operate at temperature range from -40°C to +75°C. These withstand greater shock, vibrations, surge, and EMI/EMC variance. Moreover, these contain support Virtual Chassis technology, HPoE (75W/60W) and IEEE 1588v2 PTP. In addition, OS6865 supports advanced overlay SPB-M technology while OS6465 provides advanced security through MACsec support.

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