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Why Can't I Use SUB Mode in Bridge Extension with MQTT 3.1.1?

Why Can't I Use SUB Mode in Bridge Extension with MQTT 3.1.1?

Owned by Daria Samkova, created
May 19, 2025

Question

Can I configure the HiveMQ Bridge Extension in SUB mode to connect to an external MQTT 3.1.1 broker?

Answer

Explanation

The HiveMQ Bridge Extension allows users to bridge messages between HiveMQ Cloud and external MQTT brokers using PUB (publish) and SUB (subscribe) modes. However, there are specific protocol limitations:

  • SUB mode relies on MQTT 5 shared subscription features, such as topic filters with the $share prefix.

  • MQTT 3.1.1 does not support shared subscriptions. As a result, the SUB mode cannot operate properly with MQTT 3.1.1 brokers.

What Happens If You Try?

If you attempt to use SUB mode with a remote broker using MQTT 3.1.1, the following will occur:

  • The Bridge Extension logs a warning at startup.

  • The bridge connection fails to initialize.

Supported Configuration

MQTT Version

PUB Mode

SUB Mode

MQTT Version

PUB Mode

SUB Mode

MQTT 3.1.1

✅ Supported

❌ Not Supported

MQTT 5.0

✅ Supported

✅ Supported

Recommendation

If you're connecting to an external broker (e.g., The Things Stack, which uses MQTT 3.1.1 and QoS 0), you have two options:

  1. Use PUB mode only – Publish data to the external broker from HiveMQ.

  2. Upgrade the external broker to MQTT 5 – If SUB mode is required.

Additional Notes

  • This is a protocol limitation, not a HiveMQ bug.

  • No changes or fixes are planned for MQTT 3.1.1 SUB mode compatibility due to MQTT spec constraints.