USG20W-VPN - are there known reasons for 5G wireless flapping?

pauloamgcosta

Hello community;

I recently 'inherited' a USG20W-VPN firewall and have been exploring how to use it.
One problem I'm hitting since the first moment is that the internal AP, when using the radio profile for 5GHz, keeps flapping on and off every couple of minutes, even with the simplest configuration applied via wizard, at first-time boot after a reset.
The 2.4GHz profile seems to hold up but the truth is that I haven't had the opportunity to put this equipment to any real usage so I'm not even sure of that.

I'm running the latest firmware V5.43(ABAR.0) - which now states the firewall is a USG FLEX 50W, BTW.

I tried to google around and even in these forums only came up with some 6 year old couple of threads which don't describe this behavior I'm observing.

Are there any known reasons and corrective steps to successfuly use the 5GHz band with the internal AP?

Thanks in advance for any pointers.

—
Paulo

Zyxel_Melen

Hi @pauloamgcosta

Have you checked the disconnect logs? Please share with us so we can help to find how to improve.

Additionally, you can also reference these most common causes and the recommended steps to resolve this:

Troubleshooting Steps:

Avoid DFS (Dynamic Frequency Selection) Channels

The most frequent cause of 5GHz flapping is DFS radar detection. If your firewall's internal AP is set to a channel between 52 and 144, it must legally suspend transmission and scan for radar signals if it detects any interference.

• Fix: Manually change your 5GHz channel setting from "Auto" to a non-DFS channel (specifically 36, 40, 44, or 48).

Reduce Channel Width

By default, the AP might be set to a wider channel width (such as 80MHz) to achieve higher speeds. However, wider bands are highly susceptible to interference and are more likely to overlap with DFS frequencies.

• Fix: Adjust the 5GHz channel width down to 20MHz or 40MHz to increase signal stability.

Adjust DCS (Dynamic Channel Selection) Settings

If DCS is active, the firewall may be constantly scanning for "better" channels and dropping active client connections during the transition. Set the DCS scanning schedule to a specific time (e.g., late at night) rather than allowing it to scan dynamically during active hours.

Zyxel_Melen

Hi @pauloamgcosta

Have you checked the disconnect logs? Please share with us so we can help to find how to improve.

Additionally, you can also reference these most common causes and the recommended steps to resolve this:

Troubleshooting Steps:

Avoid DFS (Dynamic Frequency Selection) Channels

The most frequent cause of 5GHz flapping is DFS radar detection. If your firewall's internal AP is set to a channel between 52 and 144, it must legally suspend transmission and scan for radar signals if it detects any interference.

• Fix: Manually change your 5GHz channel setting from "Auto" to a non-DFS channel (specifically 36, 40, 44, or 48).

Reduce Channel Width

By default, the AP might be set to a wider channel width (such as 80MHz) to achieve higher speeds. However, wider bands are highly susceptible to interference and are more likely to overlap with DFS frequencies.

• Fix: Adjust the 5GHz channel width down to 20MHz or 40MHz to increase signal stability.

Adjust DCS (Dynamic Channel Selection) Settings

If DCS is active, the firewall may be constantly scanning for "better" channels and dropping active client connections during the transition. Set the DCS scanning schedule to a specific time (e.g., late at night) rather than allowing it to scan dynamically during active hours.

pauloamgcosta

Hello @Zyxel_Melen and thank you very much for your support.

As I was experimenting with this unit I noticed I could boot an older firmware - V4.35(ABAR.2) and tried it out. 5G setup worked flawlessly from the beggining and without modifying any operational parameters.
Encouraged by this progress I ended up doing all the device configuration with the older firmware and today I exported the startup-config, booted the latest firmware and uploaded the config file. 5G is now working correctly and I have no idea what might be different from my first try.
The default 5G config with all the DCS and auto channel selection settings is holding up and in use for the past couple of hours with no issues.
I did notice however that between these two firmware versions there are differences in the AP setup, the most obvious being the absence of the AP profiles under 'objects'. The mechanics for configuring the AP seem slightly different and it might be possible that some exotic setting got carried which is not displayed under the new firmware's webui but I'm speculating.

At the moment all is working as expected, so time for some more exploration.
Again, thank you for your support!