Networking

Network Resiliency – Device Level Ring (DLR)

Part 1 – Device Level Ring (DLR)

Resilient plant-wide network architectures play a pivotal role in maintaining overall plant uptime and productivity. Industrial Automation and Control System (IACS) application requirements such as availability and performance drive the choice of resiliency technology.

When selecting resiliency technology, various plant application factors should be evaluated, including physical layout of IACS devices (geographic dispersion), resiliency performance, uplink media type, tolerance to data latency and jitter and future-ready requirements.

Over the next several weeks, we will highlight the various network resiliency protocols, such as DLR, FlexLinks, EtherChannel, and see how they may be applied in various IACS applications.

In part one of this series, we will look at Device Level Ring, or DLR protocol.


What is DLR?

Device Level Ring (DLR) is an EtherNet/IP protocol that is defined by the Open DeviceNet Vendors’ Association (ODVA). DLR provides a means to detect, manage, and recover from single faults in a ring-based network.

The DLR protocol is deployed in the ring topology and is intended for industrial automation control system (IACS) applications that require high speed convergence and single fault recovery for continuous operation.

The DLR protocol can be implemented in switch-level (IES only), mixed device/switch-level (combination of IES and IACS devices), and device-level (IACS devices only) ring topologies.


How does DLR work?

During normal network operation, an active ring supervisor uses beacon and other DLR protocol frames to monitor the health of the network. The backup ring supervisor and other ring participants monitor the beacon frames to track transitions between normal and faulted connections in the ring.

A DLR configuration includes the following beacon-related parameters:

  • Beacon interval—The frequency the active ring supervisor uses when transmitting a beacon frame through both of its ring ports.
  • Beacon timeout—The amount of time that supervisor or ring nodes wait for the reception of beacon frames before they time out and take an action.

During normal operation, one of the network ports on the active supervisor is blocked for DLR protocol frames. However, both network ports continue to send beacon frames to monitor network health. The following illustration shows the transmission of beacon frames from the active ring supervisor.


DLR Ring Supervisors

A limited number of devices can operate as a DLR supervisor. For a complete list of supervisor-capable devices, refer to the Rockwell Automation Knowledgebase Support Center, answer ID 116818 (TechConnect required to access).

  • ControlLogix 1756-EN2TR, EN3TR, EN4
  • CompactLogix 1769 L1, L2 and L3 controllers
  • CompactLogix 5069 controllers in DLR mode
  • Stratix 5700 (select models)
  • Stratix 5400 (all models)
  • 1783-ETAP, ETAP1F, ETAP2F
  • 1783-NATR

DLR Ring Participants

A ring participant is a node that processes data that is transmitted over the network or passes on data to the next node on the network. When a fault occurs on the DLR network, ring participants reconfigure themselves and relearn the network topology. Ring participants also report fault locations to the active ring supervisor.

A limited number of devices can operate as ring participants. For a complete list of DLR-capable ring participants, refer to the Rockwell Automation Knowledgebase Support Center, answer ID 49185 (accessible to everyone).

Common ring participants include:

  • ControlLogix 1756-EN2TR, EN3TR, EN4TR
  • CompactLogix 1769 L1, L2 and L3 controllers
  • CompactLogix 5069 controllers (DLR selectable in configuration)
  • CompactLogix 1769-AENTR
  • Flex I/O 1794-AENTR, AENTRXT
  • Flex 5000 I/O 5069-AEN2TR
  • POINT I/O 1734-AENTR
  • SLC-500 1747-AENTR
  • 1715-AENTR
  • PanelView Plus 7 Standard (select models)
  • PanelView Plus 7 Performance
  • E300 Electronic Overload
  • Stratix 5700 (select models)
  • Stratix 5400 (all models)
  • Stratix 5800 (all models)
  • 1783-ETAP, ETAP1F, ETAP2F

Number of Ring Nodes

It is recommend that you limit a DLR network to fewer than 50 nodes. If your application requires 50 or more nodes, it is recommended that you segment the nodes into separate DLR networks.

For rings of only switches, it is recommended that you use no more than 24 switches and 230 end devices.

If you use more than 50 nodes in a DLR network, be sure to test your application before production.

Too many ring nodes can result in a higher probability of multiple faults, slower fault detection and recovery time, and decreased network performance.


Ring of Devices

The most basic implementation of DLR is a ring of DLR-capable devices. In the following example, devices without DLR capability connect to the ring via Ethernet taps.


Ring of Devices with a Switch

In this network, a DLR-capable Stratix switch is connected directly to the ring. The switch connects the ring to the rest of the network. If the switch is configured as the ring supervisor, the switch also provides consolidated status and diagnostics for the DLR network.

Like an Ethernet tap, a DLR-capable switch can also connect non-DLR devices to the ring.


Ring of Devices with Multiple Switches

When multiple DLR-capable switches connect directly to a ring, they can serve different purposes:

  • One or more of the switches that are configured as redundant gateways can connect to the outside network.
  • One or more of the switches can connect non-DLR devices to the ring.

Ring of Switches

A DLR network can consist of solely DLR-capable switches and still support a high-speed convergence time of 3 ms or less with these restrictions:

  • Limit one ring to no more than 24 switches and 230 end devices.
  • Use only one VLAN in a ring.

Ring Speed

Devices within the same ring can have a speed of either 100 Mbps or 1 Gbps. Speeds of devices cannot be intermixed within the same ring.

A switch that supports multiple rings, such as the Stratix 5400, can have each ring operate at different speeds. For example, Ring 1 can operate at 100 Mbps, Ring 2 can operate at 1 Gbps, and Ring 3 can operate at 100 Mbps.

Make sure that all ring nodes are set to auto-negotiate speed. Do not set one ring node to auto-negotiate speed and set a manual speed on the next connected node.


DLR Resources:

CPwE Resources:

Stratix Resources:


DLR Demonstration:

Looking for more information?
Contact your local automation specialist or account manager at The Reynolds Company to discuss your automation applications.