the difference between SIM Failover and Switch SIM actions in SureLink

This article explains the difference between SIM Failover and Switch SIM actions in SureLink settings and clarifies their operational precedence during connectivity issues.

Overview

Digi devices equipped with dual SIM slots and SureLink monitoring provide two distinct mechanisms for maintaining connectivity:

• SIM Failover
• Switch SIM (SureLink Action)

Although both involve changing the active SIM, they operate under different conditions and serve different purposes.

SIM Failover

Definition:
SIM Failover automatically switches to the secondary SIM slot when the modem cannot establish a cellular connection using the current SIM.

Configuration > Interfaces>Modem> SIM failover

Typical triggers include:

• Incorrect or unsupported APN configuration
• No service from the carrier
• SIM card issues preventing initial registration

Key point:
SIM Failover occurs before any SureLink tests run because SureLink requires an active cellular connection to function.

Switch SIM (SureLink Action)

Definition:
Switch SIM is a SureLink action executed after a cellular connection is successfully established. It is triggered when SureLink tests (e.g., ping, DNS, HTTP) fail, indicating that the connection does not provide usable network access.

Configuration > Interfaces>Modem>Surelink>Recovery actions>Switch SIM

Example scenario:

• The modem connects to the network, but SureLink detects no internet reachability or DNS resolution.
• SureLink then performs the configured action—such as switching to the alternate SIM—to restore connectivity.

Priority and Precedence

• SIM Failover takes precedence during initial connection attempts.
  If the modem cannot register on the network with the active SIM, SIM Failover will switch to the alternate SIM automatically.
• Switch SIM occurs only after a successful cellular connection is established.
  SureLink tests validate network usability, and if they fail, the Switch SIM action is executed.

Summary:

• SIM Failover operates at the connection establishment stage, while Switch SIM operates at the post-connection validation stage.

Best Practices

• Enable SIM Failover for resilience against carrier or SIM-level issues.
• Configure SureLink tests and Switch SIM actions to handle upstream connectivity failures (e.g., internet outage, DNS issues).
• Review SureLink logs to confirm which mechanism triggered a SIM change.