Network Resiliency – Flex Links

Part 3 – Flex Links

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.

In this network resiliency series, we will highlight the various network resiliency protocols, such as DLR, REP, FlexLinks, EtherChannel, and see how they may be applied in various IACS applications.

This week we will look at Flex Links for redundant star topologies.

What is Flex Links?

Flex Links is a Cisco proprietary resiliency protocol that is an alternative to STP and EtherChannel in redundant star networks.

It is used to connect an access switch to a distribution switch.

Flex Links Operation

With Flex Links, you define an active uplink interface and a backup uplink interface. The figure below shows the process by which Flex Links converge a redundant star topology.

To begin, the active interface is in the up condition. The interface that is up sends and receives frames just like any other Ethernet port. The backup interface begins in the standby state. The standby interface establishes a link to the other side of the connection (that is, it is up/up by both switches).

However, the interface in the standby state does not send or receive any packets. Only the interface that is up sends and receives all of the traffic to and from the switch. When a failure is detected on the forwarding link, the MAC address and multicast entries are transferred to the standby link. When the failed interface is restored, it becomes the standby link.

Note: Flex Links does not function in a ring topology.

What Stratix switches support Flex Links?

  • Stratix 5700
  • Stratix 5400
  • Stratix 5410
  • Stratix 8000
  • Stratix 8300

Configure Flex Links on a Stratix Switch

To configure Flex Links on a Stratix switch using Command Line Interface (CLI), perform the following steps:

  1. Type the following commands, after each step below click enter to execute the command 
    1. enable
    2. configure terminal
    3. Interface Fa1/(Port number)
      Note: For Gigabit ports use Interface Gi1/(Port number)
    4. switchport backup interface Fa1/(port number)
      Note: The port number in step 3 is different than port number in step 4. The port number in step 4 will be a backup port for the port in step 3
    5. end
    6. copy running-config startup config

Note: To verify the configuration type the following command show interface switchport backup

Advantages of Flex Links

  • Ease of use — Simple protocol to manage resilient uplinks between two switches
  • Performance — Fast convergence of unicast and multicast traffic, with built-in features to improve multicast convergence
  • Compatibility with STP — As Flex Links blocks one port, STP does not identify a loop and inappropriately block any ports
  • Interoperability — Although Flex Links is proprietary, the fact that it does not communicate or negotiate with other switches means that the protocol can be used in mixed vendor environments

Disadvantages of Flex Links

  • Not standards-based — Protocol is Cisco proprietary, so it can only be configured on devices operating Cisco IOS
  • Bandwidth — Does not take advantage of the available bandwidth (only one link forwarding traffic)
  • Not configurable via Device Manager web interface on IES (must be configured via CLI)

Flex Links Resources

CPwE Resources:

Stratix Resources:

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